That's a very valuable Model 12 if it checks out. Your friend needs some expert advice if he intends to sell it.
This guy is the acknowledged expert on the Winchester Model 12, having literally written the book on the subject.
http://daverifflegunsales.com/I'd strongly suggest contacting him. There were a relative handful of Model 12s built as 28 gauge guns and some more that were converted from 20s and 16s, as all three gauges used the same frames. The difference in value is substantial.
If your friend intends to sell the gun, he should also get a letter from the Winchester records custodian confirming the gun's original features. Winchester letters can be obtained from the Buffalo Bill Historical Center, in Cody, WY. Check their site out at
http://www.bbhc.org/home/
