Further Observations:

Seeing the hollow underside of the tin-plated/bronze fixture, attached to P. Breakey's Magazine Lee- Enfield rifle, is enlightening.

It shows how the fixture was quickly attached and released from its 'carrier' on the STRM.

The 'mounting points', that I have circled in the attached photo are for mounting the 'Head', which contains the sub-target holder, marking needle, and elaborate universal-joints and rod, that move in unison with the rifle, as it is aimed at a second distant target.

There are three pairs of mounting points: Top - for offhand, Middle - for kneeling & sitting (in use in photo), and Bottom - for prone.

(p.s. - The rifle, when locked into place on the 'skeletonized carrier' of the STRM, was not weightless.
The counter-balance 'globe' was to offset any added weight of the fixture and carrier apparatus. The operator only had to deal with the actual weight of the rifle).