This model is based on the 1970s set 3 model 3.7. The main alterations are the tyres; two different types from the 1990s (I think) have been used, and the seat is changed to allow for the drive mechanism through. The steering mechanism has been omitted.
The rear axle has a ratchet fixed to it. A 57-tooth gear wheel is loose on the axle and this is meshed with a 19-tooth pinion which is journalled in a short coupling on the axle. An angle bracket, loosely bolted to the gear wheel, engages with the ratchet, there being not enough ground clearance for a pawl. This provides the model with a freewheel (as on a bicycle).
A standard postman’s elastic band has been used. This model is hopelessly underpowered and will not travel and great distance. Doubling up the elastic band makes a huge difference and results in fewer elastic bands breaking due to over-winding.