The IWM has an example of a foul anchor arm badge, but with initials A and A to each side. Google shows another with R and A.
Could be linked with Maritime regiments, RA, and could a Gordon’s TA unit have converted to RA, retaining some Highland bits of uniform? It certainly happened here with a KLR Bn changing to RE with searchlights, then to RA/AA with guns. They kept elements of title and possibly uniform.