Mamod Mobiles
Just like the stationary engine range, the mobile models have subtle features which can help the collector to fairly accurately date their model...
Until around 1968 all Mamod rollers were made with cast aluminium wheels. The 1967 SR1a was only made for about one year with aluminium wheels. Mazak replaced the aluminium type in 1968. The TE1a and SR1a both initially had straight control levers until around 1967/68(?). Only the very earliest SR1s had one-piece scuttle and burner assemblies, from 1963 with the introduction of the Traction Engine the new steel 'pan type' vap burner was used (replacing the brass type ones of the same size), although it continued to use screw fixings until 1965. The SE3 continued with a soldered brass vap lamp, althought the early handles were made of wood (white then red), later examples had a red plastic handle. The SE3 lamp also came in two sizes - very early versions being longer (and made of steel/brass) than the later all brass types. 

The earliest SRs had no tow hook. The roller has never had a canopy, until recently (2005) with the introduction of the SR1K kit. The Steam Tractor Kit (TWK) introduced around 1980 has SA1 front wheels, as does its lumber wagon. Look out for the plain fronted silver disc smoke box type TW1 which was not supplied in a kit form.
Only the very earliest TEs had a plain roof with no ribbing. Very early TEs had a special steering rod which was bent to fix into the front of the engine's axle. Some TEs and SRs had a decal affixed to the firebox base (large rectangular type). The TE lost its exhaust throttle in 1967, when it became the TE1a. Crank shafts on both engines were much thinner on the very earliest models, cranks were made of brass plated steel too (and sometimes inscribed 'Mamod') and some had the one-piece eyelet type connecting rod. Both the TE and the SR were sold in a blue picture box from 1968 to 1972. On both engines (SR and TE) export models were fitted with a sight glass earlier than the UK, hence some models can be found with a slight glass cut out in the firebox end, but still fitted with an overflow plug (1977).
The TEs rear wheel hubs were increased in size sometime around the late/early 1980s.
Both the SR and TE had no firebox holes for the SW1 chassis (which used the same firebox) until 1972 with the introduction of the wagon, it should be noted some of the earliest traction engines and rollers also had a small 'vent' situated under the boiler and cut into the firebox front wall.
Boiler paint colour has varied over the years for both engines, both the early TEs and SRs used an apple green paint, over the years this shade had become darker, until around the 1990s it had become more of a GWR brunswick green.
Like the stationaries, both models utilised spring reset whistles from 1972.
All blue SW1 steam wagons are solid fuelled. The SW1 lost its smokebox designation along with the TE sometime in the 1980s. The engine control lever can be found in red as well as black plastic, the red ones (especially on the blue wagons) is the earlier form. Some blue SW1s had white lettering on the main body, (Late 1970s).
All early steam roadsters (SA1) date from 1976 and had six-spoke 'artillery wheels' (actually SE3 flywheels!), overflow plugs and divided rear drive. Some of the very earliest SA1s were meths fired. All updated and facelifted SA1s ('Rolls Royce type' wheels, sight glass) date from 1978. The all brass SA1 dates from 1983 - only 1,170 were made, the brass finish was not very durable as the 'brass' effect was simply plated on. The green version of the roadster (late 1990s - on) is known as the Brooklands Roadster. A maroon version is available too, again late 1990s. The Le mans racer is a more complex derivation from the basic SA1, but incorporating twin pistons. However this model has no reverse control, but can be adapted for simply RC control (steering only). The Fire-engine (FE1) and the delivery vans DV1 (Green) and DV2 (blue) as well as the London bus LB1 are all derived from the basic roadster, as is the steam limousine.

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Mamod Stationaries
There are many other more subtle features that can help identify your stationary engine's date of manufacture...
The Mamod Minor 1 had a single wick burner until 1970, it then had a very narrow vap type burner until 1976 when it got a solid fuel burner which was much narrower again than the rest of the range's burner trays.
All engines prior to 1949 did not have any form of 'superheating'. When the engine frames were revamped with pressed steel, the pipework was modified to loop under the boiler giving a mild superheating effect. All superheating on the SE1/2 was dropped for the 1967 revamped models (SE1a/SE2a). Only the SE3 retained its superheated steam pipe. Some early mazak flywheels (1953) used the earlier hot brass stamped pattern/casting.
Up until around 1958 all the fireboxes were of the 'open top type'. With the introduction of the vapourising lamps across the whole Mamod range (except the MM1) the fireboxes were revised with a more enclosed top edge and a wider firebox opening. In the 1979 redesign of the whole stationary range, the firebox opening was closed with the help of a 'steel plate' modelled on a real furnace door. This item was part of the solid fuel tray's manufacture.
Forward and reverse controls did not appear on any Mamod stationary until 1967 with the introduction of the SE2a. (A reverse control had been fitted to the MEC1 in 1965 - although only made by Malins and designed by Meccano).
The steam regulator on the SE2 disappeared in 1967 when it became the SE2a.
In 1969 the only variation of the SE3 was introduced - the Griffin and George version, complete with hard soldered boiler, no regulator and specific G & G badging. All subsequent 'G&G models' after the initial run of 2,000 models were merely badged by Griffin and George, this includes later SE3s and the SP4. The original SE3 G&G was only supplied to schools.
Sometime around 1967 and continuing into the early 1970s the 'engine frame green' was sometimes replaced with a metallic 'hammer effect' paint. This reverted back the the 'Ford Highland Green' sometime after that, although this shade seems to vary from minty green to a darker sage green. I find the sage green being the later shade of paint ie 1970s. In 1979 all the range was repainted in a red/blue colourway to match the revised tool range.

The revised SP range of engines comprised the SP1, 2, 3, 4 and 5. The low sales and financial difficulties of the company meant that the SP1 (old MM1), the SP3 (Mamod badged and repainted MEC1) and the twin cylindered SP5 were dropped in 1985. A new SP5 was introduced in 2002 complete with dynamo, although basically an economy version of the first SP5. Dynamos were also fitted to the SP2(D) and the rarer SP4(D). The later SP6 with the new piston valve engine was brought in during 2006 - a limited edition version for a steam reseller in the UK had what I believe to be the first Mamod with a green base commercially available.
During the mid 1960s the SE3 started to have its decal at the end of the firebox (chimney end) - the design was the 'compressed scalloped edge type' which was also used on the MM1.
Early Mamod tools with mazak flywheels can be recognised by the 'unclipped' wheels ie the rims are left painted red. Flywheels varied in thickness, although the later 1960s and 1970s versions seem to be thinner. The power press sometimes had a centre hub goove as well as a rim groove for drive bands - probably inherited from the power hammer. Up until the early/mid 1960s all Mamod tools had lubricating holes for the plain bearings.

Finally Marine engines were reintroduced in 1958 the ME1 and the ME2 (an ME1 with wick burner was available  from 1937-40). Both used the new brass vapourisng spirit lamp with an engine block (angled on the ME1) equipped with oil reservoir (soon dropped). Later MEs used the steel pan type vap lamp and later ones still used the solid fuel tray. The ME1 had a stern tube and prop, the ME2 had a shorter baseplate, upright engine frame and dogged power take off pulley. It was dropped in 1965, in favour of the ME3 (using and ME2 baseplate) using an SEL engine - which some would say is better than the Mamod unit. About 2,700 ME3s were made before they were dropped in 1972. The ME1 soldiered on until the late 1970s/early 1980. Meths, and solid fuel types are available as are sight glass versions. All are now rare.