Further update on this. I received a response to an email that I sent to Schneider Kreuznach. Since they will longer be manufacturing bay mount filters, they've stopped installing retaining rings in the mounts. Apparently they feel that the rings aren't necessary, despite that fact that I myself have had a couple of them come apart.

I certainly won't be purchasing any more B+W Bay filters and, to be frank, I would not have expected this from a company with the history of S-Z.