Are there any differences between a Canon IIIA and a Canon IVSB?

I'm in the market for a Canon Leica look-a-like with a 1/1000th second ss. Is there any reason to buy one of these models over the other? Also, I've never seen a demonstration with the single viewfinder bodies. Is it literally just the composition window and viewfinder in the same window? Or does it work differently?

Have you check out the Canon Rangefinder site?
https://www.canonrangefinder.org/
I think that you should be able to find the answers to your questions (and many more) there.

The III-A and IVSB are very similar, the main difference is that the IVSB has a side-mounted flash synch, the III-A does not. There are a few other minor differences. As for the viewfinder, there's a built-in magnifier that will change field of view as you move the magnifier lever. This works well and is very helpful.

Go to the official "Canon Museum" history webpage for more info.

Jim B.