Somebody's either pulling your leg, or mistaken. Now as a MKIII owner, I've been told for years by well-meaning people that a "Harley starter will bolt right into a Commando". Without going into all the detail on that, it's another thing that's wrong, but a hell of a lot closer than the gearbox bit. Now, things I have seen that may lead people to believe this stuff: I saw a beautiful featherbed special many years back in one of the various Brit magazines, don't remember which. A gent took a Harley lower-end (believe it was Generator pan-head vintage) grafted on two Matchless G80 barrels and heads, and used a Norton gearbox. ( they called it The Harmless Norton!) I think people see stuff like that and think there is some kind of cross pollenization of this stuff. One popular mod I've seen in person a couple of times, is using a BSA A10 (pre-unit) 'box mated to a Harley 45 in a bobber style set-up. Seen a few of those out there. And of course, there is a local guy selling an A65 choppa on Craigslist for the last six months calling it "a British version of a Harley - loud and fast". So you can see how some of this gets out there. Especially when a friend of a friend is telling it.