As others have stated, most likely a model “C” with the rounded-corner tank, single poppet valve carb, and the starter knob on a flat flywheel.
Unfortunately, Elto did not start stamping serial numbers into a second location on the motor until 1927-ish when they started building the first Speedsters. Its possible that the model J and K motors have the serial numbers stamped into the crankcase, but I’m not aware of anything earlier having been ID’d that way at the factory.
Ruddertwins were supplied with brass timer bushings on the 1921-26 motors. The part number changed for the 1927 service twins so that may be where they started using zinc timer bearing (the parts list calls it a timer bearing) and those are the ones to be wary of, as they almost always need to be replaced if they are still the original part that was installed at the factory. Thats why the most common Elto timers that usually need new timer bearings are on the model J and K twins as well as the 1928-29 Speedsters and 1929-30 Elto Lightweight folders, as those models typically were fitted with the zinc part as original equipment.
Hope this helps.
Besdt,
PM T2
He's livin' in his own private Idaho..... I hope to go out quietly in my sleep, like my grand-dad did..... and not screaming, like the passengers in his car...
