It is quite similar, but, as you noted, it is threaded and has the tang for the derailleur.

I ran a search on Mercier in the Classic & Vintage and there is a fellow who posted some pics of a blue one which looks to be the same frame as mine - you can see that the bottom bracket cup has the same design and also sports the Vitus dropouts. I've written him to see if his also has the "M" and the "Z" in the same places.

Thanks again to everyone for all their help!