I thought they might use IBM internal date codes, but was unable to find some example AT model F kb codes to examine.
I had a page on them once, let me see if I can find it again.
These codes are also used to date other IBM items such as the vintage and well-liked IBM wall clocks.
here's a page on IBM's modern dates
http://www.cpu-world.com/forum/viewtopic.php?t=12736Hmm, looks like the IBM clock dates only go up through the late 1960's
http://www-03.ibm.com/ibm/history/exhibits/cc/pdf/cc_2407EVL2.pdfThe only other way to get an estimate that I know of is to read the date codes from the chips.
This is why I like hall effect kbs: they have a date code sticker on the frame of all of them.