If i may, this looks to me like a real old Gaffrig thru hull fitting. I bet if you take it to a maschine shop they could build you one just like it, i don't think you will find it anywhere.
I just saw a fitting "similar" to this at West Marine; it was made by Blue Sea Systems I think. It seems it is mostly for clamping over electronic cable as a deck fitting - but - you may want to give it a look. Good luck. - Jeff