770c? New and unsure

Just what I'm thinking, probably a typing error for 700c. This is the rim diameter, not the rear spacing. If it is an old racing bike it will probably have a rear spacing of 130/136mm. As Dicki said, steel frames will flex a bit, I had to prise my rear dropouts apart a bit to get my wheel in.

if it's an old road bike it should have a rear spacing of 126mm, newer will be 130mm. if it is the bike in the picture above then I'd have thought you'd be looking at 126mm. and almost certainly just a typo.