AlpineAndy is correct This is a 'home made' by stripping the glass from an original lid, adding a few bits of GRP then smothing flat. Hard work, and it will have the 310 'hump' in the middle. Maybe easier to start from scratch, at least that way you can get it all flat.
Where in DK are you? We and are half Dansk and visit home (Funen) once or twice a year.