As I recall, these where just standard rigid couplings from Ruland or others with one end tapped for the drill thread.
It is easily obtained by going to McMaster.com. You will have them on your dock tomorrow if you order by 5:00pm EST - with no extra shipping or any rush put on the order at all (at least that is how it works for me here near Detroit -- the orders are filled from Ohio for me. As usual, your mileage may vary).
PM me if you have difficulty finding the part.
Joe J.
P.S From McMaster:
61005K51
One-Piece Clamp-On Coupling Steel, W/O Keyway, 1/4" X 3/8" Bore, 7/8" OD
$ 18.16 Each
You will have to open up the 1/4" side to be the right size for the tap you need (3/8-24? -- can't recall for sure). Good Luck...