Pardner, you're not getting much action on your request, so I'll give you what I have.
The following info comes from a partial "print-out" I made from a thread on gunshop.com some years ago that also deals with your question. This undated partial "print-out" is scotch taped onto the inside cover of my copy of "The Winchester Model 12" by George Madis. Sam is addressing Chuck in the print-out.
"Chuck,
My friend who had two has sold them. And they're too rich for my blood. Anyway, if you look at any 20, 16, or 12, holding the gun so as to see the underside left frame interior, you'll note that the cutoff fits into a milled raceway in the frame. On the above three gauges the raceway runs about exactly parallel to the lower left edge of the frame. On 28s (real ones) the raceway is distinctly angled so that it runs farther from the edge of the frame as you go forward.
Nothing subtile about it. If you can compare a real 28 to the other gauges it's immediately obvious. And this is the reason Winchester stamped the rear receiver with a 28...not so as to make life easier for collectors.
Sam." End quote.
IIRC, to see the "28" marked on the rear receiver, I believe you have to remove the buttstock....but I'm not positive.
There are undoubtedly other clues to ID a "fake" 28GA Model 12, but this is the only reference I have.
Hope this helps.
