Revolver cylinder

Post a couple pictures, along with length and width measurements (Assuming you're sure it's a .357 cylinder). One of us should be able to figure out exactly what it is.

Not a bad idea, but know that they need to be fitted to the gun.

Never mind, I totally read that wrong.

Get a micrometer, take measurements & pictures, and get back to us.

You might just compare it to your 686 cylinder - they should be identical other than the metal type.

Does the cylinder have an ejector star that looks similar to your 686's and is the crane attached? Many, if not most cylinders that are "sitting around" came out of SA revolvers.

Be very careful of a used cylinder. I've got one from a 586 that looks perfect but it's got a blown charging hole from a hot load prior to my getting the gun. You'd never know it to look at it but it when ejecting cartridges, one would always stick. And believe me, it's not cheap to have a new cylinder installed.
Good luck.

It would have to be fitted by a gunsmith anyway and odds are very high that you would not be able to swap back and forth between the two. Better to just have yours cut for moonclips.