The "Half Chronometer" (Adjusted) slow train movement has an English Lever escapement with a Breguet sprung cut compensating balance and 15 jewels including a diamond end-stone for the balance. Geneva stop gear was used to control mainspring pressure and thereby improve isochronism.
Part of the full page description of "The world renown "Field" Watch"
in my 1930s Benson sales catalogue, prices ranged from £20 to £36.75
at a time when a 3 bedroomed house cost about £350.
Queen Victoria (from 1901 to c1907 "to the Late Queen" was used) but this movement is in a case hallmarked for Benson in 1920 and it clearly belongs with the movement as the serial numbers are the same, which in itself is rare with an English Benson.
As far as I am aware Queen Mary never issued a warrant to Benson and they certainly did not claim one on any of the dozens of watches I have seen from the 1920s and 30s or in the sales catalogue I have from the 1930's which does claim warrants for the emperor of Japan and several European monarchs.