The only reason I would suggest it's a factory typo is simple really....
At this assembly plant in 1970 E63 was an "N" code 383 4bbl.
In 1971 E63 switched to "M" code 400 2bbl, and the 383 4bbl became E65.
I don't know which cars received the "M" code 400's, we know it wasn't available in an E Body, but it's quite possible the 400 wasn't a popular option at the Hamtramck plant therefore employees making the fender tags or BCS's would see "N" and as an old habit, stamp the tags with the old familiar E63
Again, this is all just speculation.
I agree with Cody 1000%, there are a lot of reproduction tags out there, I just think that if this is the only "issue" with this tags, and others are known to exist, this could be a plausible explanation in my opinion.