The only differences that I'm aware of are that the X guns were initially built in CT when Marlin was an independent company and then the production was moved to Mayfield, KY, when Marlin became part of Remington. The barrel stamps are different, with the early ones being stamped that they were made in North Haven, CT, while the later ones have the Mayfield, KY, address. The serial number scheme changed along with the production facility. The North Haven rifles having the long standing Marlin scheme where you subtract the first two number of the serial number from 100 to determine the year of manufacture. The Mayfield rifles have serial numbers that start with MM.
I have 12 X guns with serial numbers that begin with 91, indicating a 2009 production date, and 6 with MM prefixed serial numbers. I haven't noticed any difference in build quality between them.
The X guns in 223 that I have seen or owned have all had MM serial numbers, while all of the stainless X guns that I've seen or owned all had the old Marlin scheme serial numbers.
I don't know what the production run for the X guns was or how many were made. I like them because they were inexpensive and easy to swap barrels on because they used the same barrel tenon specs as the small shank Savage 110 series. I put together several Marlin X gun in 22-250 with a 1-9" ROT by rechambering Stevens 200 barrel from 223. The only catch in doing the barrel swaps is that the bolt face of the Marlins is recessed slightly deeper than the Savage bolt face, so you can get a false headspace read if the front of the Marlin bolt comes in contact with the rear of the Savage specs barrel before the shoulder of the case comes in contact with the shoulder of the chamber. This results in slightly excess headspace. I know of multiple situations where this happened and in one case where it resulted in a case separation.
