Actual Item Pictured.
"B"marking indicating the later pattern but also early "BSA: stacked rifle logo on hinge.
Shallow type knurling on the handle as seen on the early type auxiliary mounts.
M/78 marking on ring
Mount for Vickers & Lewis machine gun
Identified on page 470 of Goldsmith's "The Grand Old Lady of No Man's Land" as:
534. The "Mounting, Monopod, Vickers, .303" MG, MK1", resembling that used with the Boys anti-tank rifle, Approved by LoC June 20, 1944 but first introduced in 1943, the monopod was issued to AFV crews for used with .303 Vickers tank guns in the dismounted role.
Note the swivel point, with allows the monopod to be folded up under the waterjacket of the gun.