For what it's worth, mine and every other gasket I've ever seen has shrunk with age so that it's actually smaller than the mirror mount.
DIY time. I found some rubber material of similar thickness and pliability (which I think was actually under-hood liner for a Mercedes?) and carefully cut new seals by hand. It of course doesn't have the lip like the original, but it does completely seal the mount against the door.
I wonder if anyone has reproduced these? The original one is more plastic but I recall someone had reproduced door handle gaskets for the Alfa 75 / Milan in a more pliable material.
If someone can reproduce these mirror gaskets, I can supply a brand new oem gasket as a mould?