deskthority

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 Here is the stamp from the bottom of the case

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.

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.)