Sounds like a Triumph wannabee!
Actually I'd go along with Wilko's suggestion, can't think of anything else. There is a nice picture in the Roy Bacon book of Twins showing a Nottingham Police Motorcyclist, I believe on a Plunger bike, I have not got my book to hand to take a look, I seem to remember him holding a telephone receiver with the set mounted on top of tank. Wonder if it is the tank your referring to?
There was a very nice A10 Police petrol tank at one of the autojumbles for quite a while for only £40 wished I had had it now.