There were Lots more than 6 made, more like many Hundreds, they were made for many years, the early ones had Ford 10 hp engines. Some later were also LPG fuelled. I used to see lots of them at the MOD Disposal sales, they are Not particularly rare, perhaps not often seen as they were bought for use off road on farms/market gardens and estates, they still turn up for sale on ebay, a few have been restored and road registered.
The reason no info is available is they were purely designed as a works tug for use within Naval Dockyards and shore based Depots, even though they had a Navy registration number they were Not for general road use, so they were regarded as plant & equipment, same as a forklift truck, Not a road going vehicle hence not included in military vehicle record archives