The serial number is most likely the 5 digit number that you found on the right stock panel. The 2XX (which since it is not the serial number) can now be reported without the X's. It is my belief that the 2XX is the assembly number as the locations reported are those typically used for assembly numbers.
As others have stated, the lack of a serial number will not have the ATF busting down your door and raiding your home. It is considered an antique and therefore not considered a firearm and can be shipped between private individuals and no FFL is required.
Close up magnified photos of the butt especially around the butt swivel would be nice. There may be remnants of the serial number on either side of the slot assuming that the serial number was written across the butt like the one in the photo posted by DesmoEd and not front to back like most S&W's.
Looks like a neat old piece and assuming that the price does not match the national debt, one that I would gladly add to my collection.
