Gordon
   I think all the ST/56-60 single ring Ball Bearing motors have the same rod. There is a series of non BB models also. All the ,56's I have seen the rod is marked 60.
Ed

Gordon   I have seen quite a few ST/40-46 double ring motors. Most do not heave a brass bushing on the crank end of the rod. I found this to be the weak spot on the entire line of ST motors. I made up my mind a long time ago to not use any of the double ring motors because of this. I tried using the ST/60 double ring motore years ago and the rods was the first part to show heavy wear. I would think a new p/l would use the newer rod. I have updated the .35,.46 and the 60's with Brian Gardner's  parts from Australia. All his replacement parts are basses on the later motors. I am not familiar with the British parts. When using Brian's parts rod clearance needs to be checked at the wrist pin as some pistons will hit the rod. The wrist pin boss needs to be opened up wider.

Also make sure that the P/L is the same diameter as the one being removed. There are two different linners in the old motors, thin and thick sleeves
Ed