Gee. I'll be contrary by objecting that Field Holster is too vague and also to me conjures images of a holster with a flap....

I've danced with the term "snap loop" for holsters of the general pattern - like the Bucheimer Perfect Fit, but then of course the examples pictured mostly don't have snaps.

In this area, at least, this was probably the most prevalent police uniform holster into the 1960's and still used into the 1970's by the old timers.