I have to vote with the Colonel on this squibble. A socket indicates to me that it is designed to fit a hex head bolt or nut.

As for OEM. I was right there with you, Italuminum. Not that I didn't believe the guys here who have them. Then, I looked it up. 3ttt indeed. Which surprises me a bit but the more interesting question for me is..., if we are to believe the mechanic's calipers and this is a 22.2 stem, did Motobecane fit these stems or only spec them? If so, did they use a non-French steerer tube when they were building these? Or just leave it to the LBS to squeeze them in there?

BTW Spaceman, a 22.2 stem fits in my Gitane steerer no problem, but is a bit too tight for either of my Motobecanes.