Properly fitting a gasket for 5513 Cartel Case?

[RickFlorida]

I had to get a better 5513 case so I got the midcase from MQ in Vietnam and am using a Helenarou 5513 caseback.  So hopefully the caseback is like gen and a 5513 caseback gasket will work for me.  I'll update once I test it with my water pressure device.  One  method I discovered is you can measure the OD of where the gasket goes, and then order gaskets that are smaller and then you can gently stretch the gaskets to fit tightly so that it stays in place when you screw it on.   It seems like photos of genuine casebacks with the gasket still in them seem to always show the gasket sitting snugly and perfectly in the groove.   Is that because it's already been squished in there or is it because the groove in genuine casebacks are actually rounded and not flat so the O ring stays in place?