The oldest cap and ball I have is an Uberti 1860, that I bought back in '71. That one, and all my other Colt clones, have the serial number on the bottom of the frame, right in front of the trigger guard, except an old Hawes 1851, that has the number on the bottom of the barrel assembly, right in front of the frame. Both my Remingtons have the number on the bottom of the grip frame, and my Rogers and Spencer has the number on the left side of the frame, right below the cylinder.
All serial numbers are in plain sight, requireing no disassembly to be seen. If your Pietta doesn't display a number in plain sight, it must be a very early one, and I would not see a problem on an old, un-numbered cap & ball with a cartridge conversion, because I have seen a few old "hard times" cartridge guns, made by tool companies and other machine shops, that had no numbers, and I haven't heard of anyone getting in trouble over having one of those.
