All I ever could find out was "Metal back to metal back" - real helpful I know.
You could do a search of OptCorp's online catalogue - that I how I found out how much backfocus spacing I needed with the reducer to SBIG ST series - Optcorp conveniently tell you the part dimensions most of the time
Don't bother asking Takahashi directly - it's like waiting for light from supernova in a galaxy 42 trillion light years to arrive... and when you finally get it, they will tell you to contact TNR/Tak America anyways. (though Vixen is even worse)