Sounds very old to me, even at my age. My suggestion is keep this post, but be prepared either to look for another tripod that is for the transit, or modify the tripod to fit. My old transit fits on a tripod with brass male threads of about 3 or 4" diameter.

I am guessing that the instrument is about 40 years old. I believe that you are right, I will have to build my own mounting base.

Age is well beyond 60. I'd guess closer to 100 or more. Remember my first surveying course about 1946. No such instruments in Cornell's stock of old stuff then.

Edit: This old guy now remembers the class photos on the walls of class rooms. Photos dated about 1900 or so and the classes then had the positions of each member as what part of a surveying party they held, such as instrument man, rod man, etc. Surveying is not taught at Cornell any more.

More edit:
Try his site.
https://www.engineersupply.com/hubs-nails.aspx

I saw they have at least one adapter for what you may need. A search for the model transit also may help.

I will check engineering supply. I have attached a photo of the transit level.