Stuart: the Long Flat (also offered in extra-length) BTFE was introduced with the 'Trap Package' in 1920; a Schnabel BTFE in 1922.
The Skeet Style BTFE was introduced in 1929 and later was available as a Streamline Slim BTFE in both short and long.

Dr Jim's "Production Records" show a single Field grade LRWF with a BTFE. Have you requested a Research Letter for 83434? Of course, a BTFE could have been added at a later date.

It appears that Field was removed and Custom engraved. It likely originally looked like this



If you'll post an image of the BTFE we might be able to say if it appears to be from Hunter Arms.