The stamping was done by the frame manufacturer.
I believe the consensus is that the N refers to night shift, 1 perhaps is a line no. or I suppose could indicate a particular plant (but NOT a final assembly plant).
Whatever it actually meant, IIRC, it was not info that Pontiac required to be stamped on the frame (the part no., date, and manufacturer's ID marking were required). So this was info that was useful to AOS, not PMD.
The S in a circle would be the frame manufacturer's ID mark, indicates A.O. Smith Corp. in this case.
After typing the above, I googled and found this which may be helpful.
https://www.chevelles.com/threads/19...coding.289941/
In that thread, it was suggested the N might indicate a year. I am 98% certain that isn't true. I'm not familiar with 1970 frames at all but my '64 GTO convertible also has an A.O. Smith Corp. frame. It also has an N stamp but on my frame, AOS stamped the date it was fabricated and included the year, mine was produced 10-19-63 (or thereabouts as I am going by memory), I have a pic of the stamping and pretty sure I posted the pic years ago in the '64-'65 forum if you care to search for it. Looks like the calendar year was deleted from the stamping by 1970.
I like the idea that your 276 might identify an AOS frame jig. Much like the Body jigs that were identified by Fisher Body, a frame jig no. would help AOS figure out where the problem was if dimensional issues were reported with the frames.
And it does look like Pontiac (and GM in general) required AOS to stamp the Manifest Code (your XU) in '70. Two character alpha Manifest Codes came into use in '65. The Manifest Codes in '64 were numeric and alpha so a different format but in any event, only the part no. identified the frame as it was shipped by AOS to the final assembly plants, no manifest code stamping at that time.
The history of frame fabrication at AOS is interesting. I worked for an AOS biz when the Corp. sold their Automotive Div. in the '90s. It was a big deal as the auto div. had been a major part of the Corp. for decades and represented about half the Corp.. revenue at the time as I recall it. But AOS survived.