It is about the size and shape of a typical EPIRB from a small sailing yacht. There are a lot of different models on the market, some that resemble it closely in shape, though most are brightly coloured.
However, some more budget conscious militaries just use commercially sourced EPIRBS and paint them olive.
Could be that it is a description of an EPIRB that somebody observed while out sailing in a contentious area (Falklands?) and reported to the authorities - this would explain the amateurish sketch and mis-spelling of "aerial".
The "Secret - UK Eyes B" may have been added later by people who had no idea what it was, but passed it on to cover their arses.