This I believe to be a Peugeot P-something, metric tubing, Simplex dropouts, 26.4 post.
The headlugs have that reinforcing ring, but there are four flats, two each top and bottom, parallel with and square to the head tube (pic is distorted):



They're nice flats, all the same size and they don't look hand-done, but the only reason I can think of is to have a go at twisting the frame.
A worrying prospect...