I would agree with [MENTION=333224]juvela[/MENTION] that the frame was worked on.

The lugs look an awful lot like the ones on mid-70s Centurion Super Lemans, top of the seat stays, too. Images from various BF threads:









Some Centurions did have serial numbers stamped in that location, although the format is a bit different (didn't want to include this image here as it's Big McLargeHuge):

https://app.box.com/shared/static/tg...xkbtkgn6pr.jpg