I don‘t the answer but have some ideas.

When Sedgley was building upscale custom rifles I know they were adding square bridges to Mausers .  So was Griffin and Howe and some other custom builders.  I would think that some Springfields were also modified too.  I don’t know how they were welding on the front receiver back then.  TIG welding wasn’t available until 1941 or so.  I suspect a gunsmith could not afford a TIG welder back then.   Most of the time just the rear bridge was square bridged due to the problem of reheat treating the front receiver ring.

Maybe Elmer the Great has some information?

I seemed to have a lot to say, considering I don’t Know the answer.

Frank