I'm looking at the 3.5" J frame 51s and read this from an old post:
Back in the day, S&W would fit a second .22LR cylinder to any of their .22WMR revolvers. You could order the gun with the second cylinder or return yours to the factory for the work.
Is this still the case? If I buy an old Model 51 can I send it to Smith and have them add a .22 LR cylinder to it?
I don't know that S&W will do it for you but if you get a J frame 22 LR cylinder from the same timeframe it shouldn't be too hard to have it fitted and it may just drop in. Finding the cylinder is probably going to be harder than finding a smith to fit it if it needs it. I've added a K frame 22 WRM to a M18 and a 22LR cylinder to a M48 with only minor fitting required for each. Good luck.