During the relevant time period, there were three RAF officers with the surname MacArthur. I can only find evidence that suggests one of those transferred to the Army, Malcolm in 1978. Iain's RAF service does not match that described, and though there was an Iain MacArthur who joined the Army at about the right time, it is unlikely in the extreme that they are the same person. John's history is also wrong.
Despite checking from 1960 to 1978, I can find no evidence whatsoever of any other MacArthur having a commission in the RAF.
where did it say he was commissioned as a crab? he joined as an enlisted right?
I have seen officer crab police with the "special" wings but never came across any enlisted - and believe me - you would have heard about it.