Visual back adapter question

I'm looking for a part# and where to purchase an adapter that screws onto the larger threaded opening on the back of a C11/C14 that has the standard 2 inch threads on the other side. I want it to allow a 6.3 focal reducer to be screwed onto the back of the ota. I have seen the adapter type that screw onto the larger threads to a 2 inch opening for a diagonal to be inserted but this is not what I am looking for. Any ideas?

See this post:

http://www.astromart.com/messages.asp?message_id=168412&page=3