Frank, I stand corrected, the earliest M3's (up to about 785000) did not come with the pre-selector lever as a standard. However, many were converted to take one so there are really early M3's with it. I had a couple 701xxx and 703xxx wiith it and I looked up my earliest that I ever had and that was a 7004xx which did not have a preview selector. This does not mean that there are 85000 M3's without these levers as Leica did the a lot of the IIIF's and IIIg's in that # range too.
On the M2, the earliest (from 926000) had a rewind button that you had to hold in place as you rewound, later it was made to stay put when you pressed it in. the problem was it was not very well secured and had a tendency to pop out and if you were rewinding fast, you could strip the film or even tear it. The lever rewind was a vast improvement here.
The "marker" on the filmrewind (the two dots on the later ones), initially it was a red slot, but probably only up to M3 710000 and after that it was a single dot off set slightly. The early "slot" version was not sturdy enough and a lot of them were replaced with the later version
The biggest problem with the early M3 is worn shutter brakes. there are no replacement parts for these anymore, so fixing it usually involves putting in a later shutter brake and converting it to single stroke!
An interesting piece of trivia: If you convert a Double stroke M3 to take a Rapidwinder (the intermediate drive from a M4P or M6) you have a single stroke action with the winder but a double stroke on the regular advance. With this conversion you can also use the M-winder or the Leica Motor M on the M3. A Motor M3!
