And then I just stumbled over this photo of an internal label on a Model F where a worker wrote the date next to the "Shop Date"
![Image](https://farm8.staticflickr.com/7360/13269036994_aa12a9e7b6_z.jpg)
5/29/84 = 4829
So I started adding and subtracting in my spreadsheet and had a theory: what if it's based on working production days?
There were 252 working days in 1984, with holidays accounted for.
So, let's jump to 1989. An example Model M with an internal shop date of 6096 and a case date of 4/03/89. The correlation between the two is approximate, because the sub-assemblies weren't manufactured the same day the label was slapped on the case.
So! 6096-4829=1267
1,267 working production days between them.
How many working days between 1984 and 1989?
'84 - 252
'85 - 252
'86 - 251
'87 - 251
'88 - 251
1,257
10 days difference can't be coincidental.
So!
Let's go back in time, where does shop date 0001 bring us? Well, I didn't get exact with working days through the years and used an average of 250, bringing us back to 1964.
"In 1957, the Electric Typewriter division also completed transfer of its manufacturing and engineering operations to a new plant at Lexington, Kentucky.
To indicate more accurately the scope of the product line, the Electric Typewriter Division changed its name to Office Products Division in August, 1964"
https://www-03.ibm.com/ibm/history/exhi ... ffice.html
It's the best theory I can come up with, anyways.
![Very Happy :D](./images/smilies/icon_e_biggrin.gif)