A CRIS adapter allows you to use a Silver Oxide battery instead of the M5's original Mercury battery.Can't look at it because I'm at my daughter's house and the kid blockage on their internet gate won't let me get to ebay. Would have liked to take a look. Son in law runs the servers for a major hospital, and he gets carried away.
I got my M5 on ebay also, and while it is in wonderful amazing condition cosmetically, I suspect it sat on the shelf for a long, long time. The slow speeds seem right on, but the high speeds are out of balance. It is a good thing to check for this before committing film. It shows up if you view the film plane with the back off, and the lens off, going through the high speeds while holding it up against something like open sky. One side will appear darker if the problem is there, and it gets worse the faster the speed. This is not just a Leica problem; any camera with the old cloth focal plane shutter can develop it and we are seeing lots of issues like this since those cameras are not getting any newer. It would be a very unusual seller who would know to check for this, or maybe who would want to check for it.
Since your camera needs skin and (is CRIS a battery modification? I don't know the term) anyway a CLA will repair this problem, and you may not have it anyway. I just figure that if you get a good deal on a wonderful camera, having to have service done on it is a reasonable thing to expect.
The M5 is a fabulous camera, in many respects possibly the best they ever made, at least for those of us who like mechanical shutters, a really great meter, and a great weapon to hit a mugger with. It was where I started with leica (a loaner) and I've been partial to it ever since, even though I like the others too.
Congratulations!! and enjoy it. I know you will.