The early short chambered M12 20ga guns can easily be converted to 2 3/4" ammo.
The chamber is lengthened. Make sure the gunsmith takes the 1/4" length chamber ring inside the frame into account when measuring and cutting the new chamber to 2 3/4" OAL.
The ejection port is lengthen forward approx 1/8" with appropriate radius at the corners. This for ease of rejection of fired 2 3/4" cases.
That's it.
There's no extra parts mods to be done as when converting the FN A5 short chambered 16ga to 2 3/4".

Many will say to leave the early Win M12 20ga guns as original, They are more valuable as orig. Well, how valuable are they?
The other comment often made is that the early short chambered guns are often fired with and have probably fired more 2 3/4" ammo than the short length shell they are chambered for,,and are still in one piece.

2 1/2" length commercially loaded ammo is available if you want to shoot that in the orig length chamber. It's not hard to reload with a standard press either.

The options and facts are there. You make your own decisions.