There were three numbers originally, the difference between each part being the overall length which would have been governed by the head thickness fitted to different models. 156274 being the longest.
The parts numbers used were:-
145200 TR250, TR5, TR6 to CP75000 and up to CC66569 USA
157231 TR6 from CP75001 and CC66570 USA
156274 TR6 from CC75001 in the two forward right hand side positions, where a bracket was mounted (157231 elswhere).
For reference original Leyland studs 157231 are 122.5mm long

An uprated stud 156274X is now available too.