Does look like some form of DBM.
DC measurements are likely only to see the resistances of the windings on the toroidal transformers.
I've seen these sorts of things used as 'downconverters' ahead of HF testgear in order to extend the range to VHF/UHF.
200-500MHz into one port, a switched-tuned reference oscillator in the same sort of range to another port, a 5MHz-rated scope or similar on the output-port.
Some catalogs etc here:
http://www.collection.archivist.info...nstruments+Ltd which might be useful searching ground.