I did not remove the gasket, I've replaced whole thing the spring with the gasket. I used pliers to remove the spring with the gasket from the old post and then inserted the new spring as is, pressed it down against the keg and screwed it on. The pin very gently sticks out of the double ring post, probably 0.5mm or less (see the photo), but the quick disconnect works just fine. I left the keg pressurised to 80 PSI for couple weeks and it didn't not drop the pressure at all. The keg is now full so can't really provide more detailed pics.
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I would still like to know how these two posts are called/defined! Is it by thread diameter do the have specific names etc? I really had an impression all the ball lock posts are the same, I was obv wrong.