The story takes quite a bit of unearthing, but this is what I have (except it doesn’t quite seem to tie in with your information…):
The first two R3 chassis, R3A and R3B, never seem to have received individual bodies, but in 1955 they were amalgamated into one chassis and given a rudimentary mule body (pictured below but not quite the one in your puzzle picture – the front end and wheelarches are different) and registered OWK 21.
R3D was prepared as a show chassis but doesn’t seem to have ever received a body.
Chassis R3A/B eventually morphed into R3G (not RG3 according to my info), along with a development of the mule body in my picture and the same registration number, OWK 21; that must be your puzzle picture, but I can’t see anything about a nickname for it.
R3A-B/R3G’s registration number, OWK 21, was eventually re-allocated to one of the first Michelotti Ferguson estate cars, R5/1, which was nicknamed the Bread Van.
I haven’t been able to find any other nicknames for any of the Ferguson cars after the Crab, but after your clues I was going to guess ‘Land Ferger (or Fergie/Fergy)’ (and note I’m writing this on Saturday whilst the puzzle is locked for Wendax but I’m guessing he will soon be solving it!)