[RogerBW]

Familiarity is oddly defined, but it seems that the default assumption is that a different model of a thing should be a different skill – the example of "two similar models of Colt revolver should be considered identical" is the guide I'm using here.

Something like Mechanic (Heavy Airplane) is already a game convenience; technical crews have their own specialists, and engine repair is quite different from bodywork repair. For that matter, repairing a Merlin engine that was mounted in a Spitfire really ought to be the same skill as repairing a Merlin engine that was mounted in a Lancaster - but if the ground crewman chose to learn Mechanic (Heavy Airplane) rather than Mechanic (Internal Combustion Engine) he's has no default from one skill to the other, never mind Familiarity. I'm not convinced that the vehicle-type Mechanic skills really work with Familiarity except on a case-by-case basis.