Model F date markings

[dotdash]

I recently picked up an F XT fairly cheap (It's a bit rough, both feet are broken, and the DEL key is missing)
The plan is to clean it up, remove the AT cord and wire in a Soarer's to a F mini jack. The plate shows some rust spots, but I was going to forgo ripping it apart if it worked ok. I came across a stamping on the case and thought it might be a date, it roughly corresponds with shop date info. Is anyone familiar with the markings on an F XT?
Here is the shop date, which has another sticker visible below it

flabel.jpg (186.11 KiB) Viewed 1099 times

Here is the stamp from the bottom of the case

fcase.jpg (51.5 KiB) Viewed 1099 times

[Chyros]

Somebody wrote a piece on it here on DT. I think it had to do with days counting from 1981 or something along those lines.

[AJM]

I also would be very interested in how these US stickers can be deciphered.
(Why couldn't they just put a proper date on it - as on the UK-made keyboards.)