About right, Lloyd. There are more built-ups than originals.

I don't think you understood the raceway. We are not referring to the shiny 28-stamped cutoff. We are talking about the milled raceway in which it lives. On 20s and 12s the raceway runs parallel to the bottom of the receiver. On 28 ga receivers it is angled in an upward (back to front) direction. This is quite easily seen on the second pair of M-12s placed together in that thread. Here's another thread w/pics of a 28 and a 20: http://www.shotgunworld.com/bbs/viewtopic.php?f=60&t=229159&p=1916241#p1916241

You can also see the shellplate stop is beefier than on the 20.

Winchester did not stamp the receivers "28" for no reason. They weren't thinking of collectors. It was b/c the frames are milled different than 20s and 16s. Understanding these differences is the most bankable way to recognize an original. Because.......well.......parts is parts.

And, to repeat, there is nothing wrong with conversions per se. In fact, they are somewhat more reliable feeding than originals. They're just not worth $10-$14k.