Rega arm mounting question NEWBIE

I recently purchased a ingonito wired RB 250 to use on a used P25 that I bought without the arm. I am having problems figuring out how to mount the arm. It did not come with any type of mounting nut to attach it to the table, and even if it did have one, I don't see any threads that could be used for that. Help! What am I doing wrong, and where could I get the parts to mount this arm?
Normally, the base of the arm platform has about a 1" diameter brass tube that is threaded, and goes through the plinth. There is a large nut that screws onto it, and secures it to the table. The cable exists through the bottom. If there are not any threads on this, then there is some other kind of modification done to this arm. I have no idea what to suggest in this case. Perhaps you need to ask the guy you got the arm from how he secured it to his table.