RX: "So, it's not just a 'stick shift camera'; it's actually like an old car where you have to adjust the mixture and advance on the fly, too."Yes, the user interface on the Leica III is pretty foreign if you're used to cameras with swing-open backs. It's positively eldritch if you're used to auto-winding film cameras where all you have to do is make sure the tip of the leader is aligned with the orange marker when you swing the back closed.
Yeah, and Robert Capa did that in a shell crater. At night. Under mortar fire. Suck it up, buttercup.
While it's possible to ham-handedly put a thumb through the shutter of a conventional 35mm film SLR while loading it, you have to be exceptionally maladroit to do this. On the other hand, loading an old Barnack Leica wrong will very likely wrinkle the shutter curtain, and then you're hosed to the tune of a few hundred bucks.