Saving Old Seagulls

Having a problem reassembling a century plus gearbox. I,m left with a gap of approx 1.5 mm where the outer casing gasket fits.Are the gaskets really this thick? Inside there is a washer, ( thrust washer?) that prevents the shell from closing up any more. I can't find a parts diagram to check. Can anyone help? Red. Cheers

Unfortunately not my bailiwick of expertise.

Hope this helps: Left click to expand or reverse pinch on phone.

Thanks for the drawing, very helpful.