mk1-forum.net

My assumption is that the foam was there to hold body filler while it cured but anything is possible. The side that did not have a hole also did not have foam. If it was from the factory, maybe it was removed while someone dealt with rust holes throughout the roof?

Yes, the foam is a factory fitted feature, intended to seal the cabin from air leaking in from the engine and boot area.
But it traps moisture and promotes rust.
So remove it when you can.