"Its base is configured to slip in from any direction".

I think this is the answer. I still think I will take this and the collar to a machine shop and get them to pin it on place with a couple of small screws.

I just purchased this the other day along with an MP-E 65. A nice solid rail but clamp orientation is incorrect to use with a lens tripod collar. Someone in another forum suggested to buy this. It works but then I realized I spent all this money on a precision rail but there is no way to verify I am...