Hi Norbert, Welcome to the Forum.
The bike frame ZA7S series is a sprung type frame, that is what the S means. In this case spring plungers instead of a rigid frame. The Z and the number tells you the year. All frames of this design are marked A7. The YA7 number would be from an earlier rigid frame.
The engine number is a similar system , an S means Star Twin, the more sporty and upmarket model.
Your frame numbers ZA7S 3709 put it at around 1950, but earlier than the 1950 model year where numbers start at ZA7S 6001
Engine number ZA7S 1489 is contemporary with the frame, 1950 model year starts at ZA7S 4001. So actual year of manufacture likely to be 1949 for both. YA7 2313 is a standard A7 engine number, again from an earlier date.
The Y numbers seem out of place. Original frame number is stamped on the left hand side of the steering head frame lug, engine number on left side of crankcase, at the front side just under the cylinder barrel. Y numbers are before 1949. The engine is a late Longstroke type, with the extra lower centre bottom bolt on the timing cover. Earlier engines have just two, and the timing cover has a flat bottom. Plenty of scope here for mis representation, so be cautious and check everything.
The bike has been rebuilt to a high cosmetic standard, retains the correct earlier smaller front brake and is fitted with a later type front mudguard. All in all it looks a good example.
Check the number of teeth on the rear sprocket. 49 teeth mean the sprocket is from a low geared sidecar bike. 46 teeth was standard, 42 teeth can be fitted for more speed or higher geared cruising.
All frame parts interchange with later models, but the engine shares few parts with the later designs. The magneto is marked with the month and year of manufacture. The carburetter fitted is a modern type, not the original remote float bowl type. These Longstroke type engines are prone to camshaft wear, regular oil changes are essential.
Gearbox looks to be the correct early type, with no grease nipple for the clutch lever pivot. Here again, most parts interchange with later variants, but the gears and layshaft have detail differences if replacements are ever required.
Swarfy.
