Will it slow me down?
So, you decided to buy the legendary Leica M3 and after reading review after review, you decided what you really need is a late model M3 single stroke (SS), with a serial number above 1 million - right? Thing is, prices are on the up for such cameras. Usually only the double stroke (DS) models crop up at bargain basement prices. My view of these cameras is as an engineer and not as a photographer, although I do use them. I work on these cameras every day, so what I have to say may be useful.
First of all, let me tell you - the DS is no different to use than any of the other M cameras. After a few shots, you just get used to it. It doesn't tire your hands and it is unlikely that it will cause you to miss any shots. The mechanics of the DS are the same as the SS. The only difference is that the wind gear has a restrictor, limiting the amount of movement. What you have therefore are two half-strokes rather than one full stroke of the SS.
So, there is no special gearing going on inside, just a restriction on how far you can advance the film per stroke.
It is possible to convert a DS camera to SS by replacing the winding block, usually obtained from a donor SS camera but this is likely to be an expensive job and my advice is - don't bother. There isn't really any advantage. The remaining 'oily bits' inside the camera are pretty much identical to any other SS model.
Rangefinder
The other point about the earlier M3's - and this is often overlooked, is the rangefinder. The DS rangefinder is very different in design from the SS and subsequent Leica models which are sometimes known to 'flare' under certain lighting conditions. The simpler arrangement of the brightline mask, without the auxiliary reflector can make for a superior viewing experience however... if you require focussing down to 0.7m, this may not always be possible on the early M3, whereas on the SS model, this can usually be fixed by an adjustment. Notwithstanding the potential problem of balsam issues inside the rangefinder - something that can affect any of the M cameras, in my opinion, the early M3 DS rangefinder is superior to that of the M3 SS or M2/4.
Frameline Preview
Some of the very early DS cameras don't have a frameline preview lever. They still have framelines selected automatically by the lens - 50mm, 90mm and 135mm but no means of a preview. Personally, I never found the need to use it but I suppose others may disagree. Like the more expensive washing machine with extra buttons. We tend to think that we'll pay extra for extra functionality that we might need. In reality, we may never use them. 'Snake legs' is the term sometimes used.
Shutter Speeds
On older cameras, before shutter speeds were standardised - 1/60th, 1/125th etc, you'll find older style shutter speeds of 1/50th of a second and 1/100th instead. This bothers a lot of people but in practice, any error is usually well within the latitude of most films. Unless your camera has been serviced and calibrated - who knows? Your camera may be firing at 1/125th and giving you 1/100th of a second exposures anyway! If it does bother you, some M3 DS cameras have the later standardised shutter speeds - go for one of those.
Strap Lugs and vulcanite
We often get asked to change the strap lugs on a later camera but those early Buddha ear strap lugs were built to last! On an early camera you may also have to re-skin the camera. This is a cheap DIY job and well worth doing. Old flaking vulcanite is toxic for your camera - just make sure you order the right skin or send it to someone like us to do it for you.
Glass Pressure Plate
The pressure plate ensures the film is held flat at the focal plane. Glass seemed like a good choice - optically flat with good wear characteristics. Such pressure plates can be recognised as it is surrounded by a frame that holds the glass plate in position. Leitz had discovered that in some cases, static can build up in use, leaving static discharge patterns on the film. The pressure plate was later changed to an all metal design. Are you prepared to risk this? I've never had any static problems with such cameras and so far, we haven't had a single camera in for repair with this problem so don't worry. It's unlikely you'll have a problem either. If you do, it's an easy fix.
The 'L' Seal
Finally the 'L' seal. Many cameras are sold with the 'L' seal intact. This demonstrates that the camera has never been opened and tampered with, which is fair enough but it also means that your 60 year old camera has never been serviced. If you buy such a camera, get it serviced properly. Lack of lubrication will cause damage, not to mention the fact that the shutter speeds will be all over the place. If the camera has been so-called 'CLA'd' - then you need to ask 'by whom and to what extent?'
I hope these notes from an engineers standpoint have been useful. Above all, grab a bargain, get yourself an M3 DS and enjoy owning and using one of the most iconic cameras of the 20th century.