The number 28 stamped on various parts - including mag button and cutoff - is important. But what you really want to know is whether you have a 28 ga receiver in your hands, and not a 16/20 that was built into a 28 ga gun using 28 stamped parts.

It's really pretty simple: http://www.shotgunworld.com/bbs/viewtopic.php?f=60&t=391354

I consider the cutoff raceway/shellplate stop more reliable than the 28 stamped on the back of the frame. What does it take to remove about .020" (to preclude ghosting) of metal from a nice flat surface, then restamp a 28? Welding/recutting an internal raceway and welding up and reconfiguring the shellplate stop seems like a whole 'nuther level of difficulty.