My FLASH bg is 723 x 647 on 4.5mm glass.
Sounds like it was printed onto wrong size glass, but the registration should all be correct from one corner.
Take the glass to a leadlight shop - theyll nibble the edges off for you. 9/10 it works out way cheaper than a glazier.
My guess is that this was a "test" run of prints - the size of glass is unimportant as they are never really intended for sale... what is important is that all registrations line up for each layer of print.
What you have there is a seconds glass, should have been sold as such though
