People will,indeed, quite often remove insignia from a real cap-especially when the original is badly mothed or damaged. It is a simple matter to save a Kreigsmarine tally from a cancered out cap and slip it onto, say, a post war Navy cap.... The cap in question is iffy. Normally, you expect to see a bluish almost purple lining with a celluloid diamond, but there are, apparently, Some like this one that did not. Personally, I would pass on it out of caution and wait for a better bulletproof cap. They are out there.
"Much that once was, is lost. For none now live who remember it."
What is the stiffening material inside the band ? This should be celluloid on issue caps but the way the cap sits flat looks like there is no stiffening. Also looks like a one piece cap. Typical issue caps have the top removable.
The correct lining colour for an issue Matrosenmütze is black, not blue so that much is correct at least. Sweat diamonds are never found on original issue caps. Coloured linings and sweat diamonds are only found on some private purchase caps but the overwhelming majority of Matrosenmütze have black linings and no sweat diamond.
Hard to say for sure from the posted images as to whether it is period or early postwar, but certainly not a "textbook" official issue cap.
Yes, you will always find manufacturing variants once you factor in private purchase pieces, foreign made pieces ( and KM headgear was certainly made in France, Norway and others) etc which is why I always only ever went for official issue pieces. With them there is no "gray area" and fakes are easy to detect. Regulation issue pieces don't need any explanations or stories so they are always the safer buy.
Even if you are convinced enough with a "variant" you will always have to convince a potential future buyer if you sell in the future. Regulation issue pieces sell themselves.
I too would approach this one with caution. The lining doesn't look right and the overall shape of the hat does not seem correct either.