I Learn something new every day! I have my Dads 12 ga Mod 12, ser. # 291XXX, 1922, that I've been shooting since the late 50's, hunting , trap etc. just looked and the #d nickel steel bbl is not ctg length marked. Always shot modern 2 3/4 shells. Maybe it was buzzed early on? 2 3/4 shells drop right in.