Bob,
The left barrel is 20 ga. and the right barrel 28 ga. The only M21 they ever did that way and only because the customer was such a prolific purchaser of M21s built for him by the custom shop. He had well over 60 M21s built by the shop. The gauge is inlayed in gold on each breech of the barrels and of course each barrel has the rolled verbage common to Winchesters which also specifies the gauge.
I was told this gun was the last M21 order the custom shop took in but was not the last one built/finished. Sorry to have missed that title!
