I don't know about any secret marks on them. I do know some model 53 Jets that came with the 22LR cylinder have the revolver serial number on the extra cylinder.
Back in the 70's I sent a model 53 back to the factory or a warrenty station (it went through a local dealer). It was fitted with the 22LR cylinder but does not have any numbers on it. I also have a 48-4 with a 22LR cylinder. It also has no numbers on it.
No secret way I know of. I have Mod 53s and 48s with aux 22LR cylinders. Some have SNs, some don't. I really think it was left up to the guy/gal who was doing the work. That's part of the mystery to S&W, in most cases, it seems nothing was absolute.
Dave