Welcome to the forums. Sounds like you got a super deal on your K1/K3 for $200!
Your serial no: CB750E-1072485 is a 750 K1 engine and it’s not a European market engine per se. All K1 engines share the same part numbers for the crankcase, head, cylinders, covers, transmission and internal components for the USA, Japanese, European and General Export bikes.
However ... part number changes that made 750 K1 European market specific were the different exhaust pipes and carburetors fitted to them. Examples of these carb/exhaust differences are found on the Germany, France, Netherlands and European Direct models. Perhaps this could be what the previous owner meant by saying that it has a European engine?
The 750 K3 was sold only in the USA / North American market which is why there isn't any Japanese, European or General Export market models.
I don’t have any information on your 1978 K.
This K1 engine & frame serial number chart below was published by Honda UK: