Here's what's underneath the plate. There's no 5/8-11 thread, as expected: I removed the tribrach for this picture. The top of the tribrach look...

I am now able to post pictures. This is what the bottom looks like: and thanks to the response above from jhframe I think what I need looks somet...

Yes, that must be it, though the design is quite different. I presume that would mate with something like this: <a href=" removed link " targ...