Interestingly, the advert for the company shows the address is near Leeds. AFAIK (not a lot admittedly), the Whitehead company that was heavily involved in torpedo design was based in the south of the UKicon, IIRC in Dorset, on the coast near Portland. I s'pose a coastal location is not rocket science to deduce either, for the location of a torpedo designer......!

But which company, if either, designed the mounts for the rifle? Or, were they two subdivisions of a single larger concern?

I have no idea, but hats off to whoever can shed more light on the subject.