[MENTION=333224]juvela[/MENTION]: good eye, the frame is indeed a 57 C-T. Top tube is 56, C-C.

About the lugs: when looking for the frame builders who built for Willem Metz, the shop owner, two famous names keep popping up: Jasper Bouma and Jacobus de Jonge (a.k.a. Witte Ko). They may have been ambitious enough to file their own shapes from existing models.