I have a feeling that it isn't a Spec An, but more like a sweeper with internal detector & display-----somewhat along the lines of a Polyskop.
Or, possibly, a Group Delay tester for microwave links?
It definitely would have been from my time working at the PMG's Dept, but, although the whole look of the thing is familiar, I can't quite place it.
Most of my time was spent in Radio & TV Broadcasting, rather than Comms, & the time I did spend in that field was mostly HF.