I had a nut like that on my Conn (from Matsumoku) from the '70s. The joint on mine is a sliding vertical dovetail under a glued-down fretboard. I believe some Sigmas came from the same factory during this period, and yours could well have the same joint. The nut/bolt hold the neck in place during gluing.
I reset mine - here's my thread on that -
https://www.acousticguitarforum.com/...d.php?t=484657 In it, there's a link to my photo gallery of the joint from various angles.
I love my Conn, and future resets will be a breeze, as there's no need to re-glue the fretboard with the vertical joint (no fret buzz up high, either, even with 4/64 on E), and I kept my intonation because I didn't shorten the neck with a cut. I did add another bolt/nut (via the insert shown) to the existing bolt/nut just to keep things tight.
Also, I didn't steam it apart - I used a clothes iron (mostly on the fretboard extension) and [CAUTION ALERT] a heat gun. I made a foil-covered cardboard surround so I could keep heat off the wood and binding, but it would be very easy to burn and/or melt guitar parts. It took a lot of time and finesse to do, with lots of pulling/heating/pulling/heating, but I love the result.