It Could be any number of things,guns can be prooved in @ 1/8" increments. I Have in fact seen 2&5/8" proof marks on a top tier London 12 ga. So here goes.
1. Meant to be 2&7/8" but reamer was a little undersize so marked as 2&3/4"
2.Mis Marked at proof.
3.Actually Built as 2&3/4" per customers request.
various others


Hugh Lomas,
H.G.Lomas Gunmakers Inc.
920 876 3745