I'm not sure about the serial number. I understand the 3B engines should start with "4J" unless certain circumstances occur, i.e. a replacement engine, etc.
https://cj3b.info/SerialNos/SerialNosEngine.htmlHaving said that, I wouldn't be surprised if your engine was a donor from somewhere else. Since I notice a few other numbers in the list having a "P" it makes me wonder what the relation could be.
As far as the studs are concerned, I've only seen bolts. I know some older jeeps had studs, but it wouldn't be hard to imagine a previous owner using studs because that's what he had or thought additional maintenance later on would be somehow easier.