Capnahab,
It may be too late now as you said you were going to open your's up, but for what it's worth, my model is I think earlier than yours, I have two small slotted head steel screws instead of the roll pin. They are very tight, I have not pushed my luck since they would be easy to chew up.
They are screwed into a round brass closure plate slotted to guide the MT bar
At the heavy end, under the threaded steel plug, there was just the other end of the MT slider bar with what looked like a plain brass washer fitted to the end, acting it seems as a guide in the plain bore; no compression spring as I had expected. The spring must therefore be a tension spring acting from under that top washer.
I would be interested to hear what you find. As others have said, what a useful tool it is.
Brian