I found a 27.2mm seat post and couldn’t get it in very far, it was tight and starting to scratch the post, a 27mm post fits good, but I’m not sure if its too loose. There’s either a burr, which I cant see, that scratched the post or its meant to be 27mm.

Looking through the bottom bracket shell at the inside of the down and seat tubes, and through the head tube at the top tube, I can see the frame was pinned together with what looks like nails.