There are several items to discuss;

1.  Is the barrel marked “1912” ?

2. The serial number you mention (342869) is a March 1923 production gun.  Accordingly, it should be a Model “12”.

3. When you measured the barrel length, did you include the chamber?  The correct measurement is from the bolt face to the muzzle face.  The standard barrel length for the 20-ga guns was 25-inches until September 1926 when it was changed to 26-inches.

Bert