Ref 6426 crown gasket?

[mechachia]

I just got a 6426, and wanted to replace the gaskets. I believe this is a twinlock crown, so there should be a gasket in the tube and in the crown.

When I took the stem out, there was a tube gasket present, but no gasket inside the crown. Not surprising for an old watch, I thought. I'd gotten a 6mm flat gasket from Esslinger, which I think is the right size, put it in, it looked like it fit, but it would not screw down with the gasket present.

The caseback gasket I also got from Esslinger worked great.

So... Did this watch not have a crown gasket? Did I get the wrong size? Maybe there's a trick to fitting it?