How to find out gasket number for Seamaster 396.0900?

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I opened my new to me Seamaster Jacques Mayol (366.0900) and I read that the caseback gasket should be replaced once the watch has been opened.
But even after some serious research I have been unable to come up with the part number for the gasket (or at least the size).
Maybe somebody could point me in the right direction.
Here is the watch in question:


Thank you for your help
 
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198ZW0050
Thank you very much for the part number. I did find it on CousinsUK. Does anybody know where I could get one in the US?