Just bought a 1972 Commando with another twist. It looks like the nameplate has been put on upside down with the number prestamped before assembly. When they came to stamp the date, they have realised that it is impossible to reach the date box so have added it after the number. This bike was laid up at the end of the seventies and is a bit of time capsule and doesn’t look like it has been interfered with.
British bike so a bit rusty.
It's is my guess that the date was stamped later as well but I don't believe that your date was stamped on the normal assembly line. Here is the original label from 201251. It was move to another frame after a crash in 1973 but it is an original factory certification label.
Notice the the date font is the same size as the serial number but not as bold and that it is "DEC 1971". Also, it is stamped in the other direction so could have been done during assembly. Norton seemed to have been trying to make up their minds how to stamp them More common is like "6 73". But you'll find lots of different formats and label orientation left or right of center and date readable in the same or opposite direction.
This one is an early 850:
Notice that the serial and date are in the same orientation and that the font and weight of the punch is the same. My guess is that each individual number of the serial and the "6" of the date were individual stamps and that the "73" of the date was a pair of stamps held together. Also that they were stamped by a single person in a single sitting.
There are quite a few threads here on this subject and this one captures a lot of it. Unfortunately it appears that any records Norton had on the stamping policy are long gone and there are not nearly enough example pictures of known original labels on frames to be sure about much.
I would love to capture this info in my Commando Survey but getting accurate info without pictures is hard for this subject so I translate the dates provided to date like "Sep 73" which seemed like the most prevalent format when I started but "9 73" seems more prevalent now.
https://www.gregmarsh.com/MC/Norton/CommandoSurvey.aspx
The only two things I'm sure of:
The orientation of the label, the orientation of the date stamp(s), and the format of the date stamp(s) changed over time.
The certification label was on the frame before the frame was on the assembly line, at least in later years - I have no idea whether it was blank, partly sampled, or fully stamped when put on the frame.