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I've noticed that some datecodes show up a lot, like 1230, 1302, 1143. It seems to me the datecodes changed more with some revision than with the dates.
The common serial number explanation is that the first two digits are the year of manufacture (minus 1960), the second two are the week number within that year, the letter indicates country, then the last five digits are the individual sequence numbers. So, the first four can be called the date code, and as you say, some date codes show up a lot. I have no direct evidence, but I do not believe that hp changed the date code at the beginning of each new week. I think it was changed only when certain events occurred, such as:
1. they produced 99,999 copies with a particular date code (assuming they would not produce a unit with a sequence number of "00000".)
2. a fairly major design change was implemented, such as the updated roms between versions 2 and 3, or the keyboard design change between versions 3 and 4.
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Also some early HP35's have a 2 piece center plate with 10 screws and later ones have a 1 piece with 8 screws. Anyone know when this change took place?
My version 2 units are all 1143A and all have the 2-piece center plate with 10 screws. My 1302A version 3 has a single piece center plate, and presumably only 8 screws, although I have not disassembled it. Does your 1230A unit have a 1-piece or 2-piece center plate? If it is 2-piece and if I had to conjecture, I would say that the 2-piece to 1-piece center plate change coincided with the version 2 to version 3 change.