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Look at what I found!

https://www.cj3b.info/SerialNos/SerialNosEngine.html

"Some Jeeps have engine numbers which are clearly not original, and were taken from another vehicle. Numbers beginning...

Here is a pretty good discussion of numbers on CJ3B's, but as you will find from the article, there was no consistency or reason to the serial number marking on anything other than the actual vehicle...