Plate and Bracket Assembly No.(whatever), I think.
It's the general-purpose top plate for demountable sets - usually on Carrier No.25 or similar - and some vehicle stations.
It attaches to to top of the set with four screws into the flat plates on the set case. The variometer is attached with three bolts from underneath (and the usual packing piece). The multitude of pre-drilled holes are symmetrically arranged so you can use it for left or right-handed installation in a 2-set vehicle (e.g. LCV).
I've got one somewhere but it's currently inaccessible due to the (completed) house move and a pulled muscle which means I've effectively walled off about two thirds of the new house with 100+ bananana boxes that I cannot currently lift.
Photo's and measurements available once I can shift stuff around.