I've had an opportunity to check out my SWC and I can confirm that there is a very small amount of play - just like you have described - between the back and body. It's not something I've ever noticed before, and I've never had any issues with light leakage when shooting on this camera. This may perhaps be normal, and if its the same with all backs and there are not light leakages, then it's probably nothing to worry about. If you're really concerned have it looked over by a Hblad repair specialist.