As far as I know, all MV cabbed vehicles seemed to have the set forward axle, so presumably they didn’t require a prefix number. As for the sleeper MW cabbed units, I only know about the 4, 5 and 7MW versions. I believe the 3MW was a rare NZ export variant and I’ve never heard of a 2MW. In any case, it doesn’t pay to seek too much logic in ERF’s classification systems, which were apt to change overnight and were not always entirely consistent!