42 4855 is the 1961 part and it is about 12 inches long.
Sorry I've been out all day so first chance to reply
Right I've bought this part twice and each time I've got one about 12 1/4 inch long, the one on my bike looks authentic ( it hasn't been cut ) and it is 11 1/4 inch long and fits perfectly, if it was any longer it would foul the drive side exhaust pipe.
The good news is I have got the original one out, it was gunged into the long hanger on the drive side, penetrating oil I've plenty of , time I've plenty of and patience (um increasingly in short supply).
Thanks for confirming this part I bought is correct, Bought from Barleycoen BTW, excellent service and they do specify that they can supply the rod at other lengths so there's is the answer I guess