[Graham]

Thanks for that ^^^ link, Kevan.  

I know that the RK- is slightly shorter than the RX-, my concern is the (potential) speed difference between the 41 and 42 both in the RK type.

I think you will find the motors were made slightly differently with time, the more recent versions were made more cheaply to save costs. Certainly the fixing of the brushes on the older motors was more robust than the more recent ones.