Mike:
Dwight and I discussed this today, and I told him I would measure the thread size that fits into the cast housing. The outer diameter of the knurled adapter is 1.254", and the closest fit I could find on the pitch was 13 tpi. A 1 1/4 UNF thread is 12 tpi and the actual diameter would be less than nominal, so I suspect the thread is metric, possibly M32-2. Even this is an odd size - couldn't find it on McMaster Carr.
I bought my threadmaster on Craigslist several years ago, with a 10 tpi leadscrew. Last year I purchased a 16 tpi leadscrew from Lee Valley as a special purchase/closeout. The photo shows the 16 tpi installed, and the 10tpi setup is shown in the foreground. Each leadscrew comes with a matching set of knurled threaded bushings as shown. This configuration is designed to eliminate slop in the leadscrew. This might give you a better idea of what it looks like.
Unfortunately the manual is pretty sparse, and does not contain any dimensions for the bushings. Bill is right, the operation is pretty straightforward, the manual doesn't add much.
The first thing you might do is contact Lee Valley. The parts no longer show up on their website. However, I checked my records and at the time the catalog number was
03J9232, with a product description "16 tpi Spindle-1"x8tpi". I bought it in June 2016 at a price of $39.30.
We just incorporated this tool into a dedicated setup with a 60 degree milling cutter in a router. Another option to consider when you get the leadscrew worked out.
Richard