Those are Campagnolo 1010 dropouts. That might be the serial number below it. Are there any other stamps on the frame, such as on the seat tube near the bottom bracket or on the bottom of the bottom bracket?

It looks British to me, but I can't be certain.

The Universal centerpulls and levers might winnow down a window, too.

Can you pull the seatpost out to look for the size? I would guess 27.2. If it were something else, that would be a clue. I would also guess that the steerer tube has a 531 stamped on that. You'd need to pull the fork.