Yaay, a workingZeiss gear for sooo cheap!
Good luck finding proper screw BTW - having whole bag of spare parts, I'm very happy to find anything useful in it. There are no common sizes in this business. I'd start with camera repair shop to get the right screw, really. You'll pay like 10 crowns and that's it, done.
If the bellows are not hollow and the missing screw doesn't let any light onto the film, I'd say shoot with it, I know no better way to check for light leaks. I think you can also release the shutter with a cable release mounted onto the lens/shutter part of the camera, it's the right solution if you use a tripod anyways. There should be a threaded socket somewhere there to screw the cable in. (Note that Nettars may have older, wider 3/8 inch tripod thread, so you'll need an adapter to screw it onto modern tripod with 1/4 inch thread...)