I'm not certain about vintage 3 ttt stems but be aware, the Cinelli from about that era took a 7 mm hex wrench. It's possible your 3ttt Record binder bolt may be the same.

It's probably not the original stem. Not sure without looking but that model was probably spec'ed with a Philippe or Atax or something. Anyway, if it was original it certainly would be a French 22. 0 quill, I would think. But like I said, the top nut on that Stronglight head set will often take a 22.2, and sometimes the steerer tube will too.

Did the LBS mechanic not try to loosen the binder bolt hex head for you ????