Jim, you might want to look at the race record of 1075, which is in John Allen's book and others. The Mark IV was only campaigned for one season and only in a few races. I think it did win everything it was entered in- they were very reliable and fast cars that ran like trains. And by that time all the teams had their acts down pat and there weren't a lot of errors. The Mark IIs were still Abbey chassis modified for the big engine drivetrain. They also won a lot of races and all the development work on the engine and transaxle benefited the Mark IV, which uses the same power unit.
Even the Mark IVs had RHD cockpits, so the position of the steering wheel doesn't mean much here. IMHO, the facts state that the Mark I-II cars are at least British-American design/builds, if not chiefly British (my feeling is that they are really British cars) The Mark IV, OTOH, is an American car, designed and built here, and driven mostly by American drivers, for what that's worth. The real shame about the Mark IV is that its racing career was so short. I believe it had a lot of victories left in it when the type was retired from competition.