I found it!
Copied and pasted from a post to the HPAK mailing list explaining the serial numbering system; the original author worked for HP:
Having worked as a Production Line Engineer, Production Line Manager and Product Support Manager(Americas) for one HP Division, please let me clarify the process for Engineering Changes and how that affects Serial Prefixes.
1. The Basic Identity of an Instrument is the Model No. (e.g. HP 8510A,HP8510B,HP8510C). These Models are all independent of each other and each of the three examples would start with a Serial No.(Suffix) of 00101
2. The most Major Engineering Changes to an Instrument (e.g. Addition of a bunch of New Features or Upgrades) could be implemented into a New Model No. (e.g. change from "A" model to "B" model). In the case of changing to a New Model No. the Serial No.(Suffix) will start again at 00101.
Short of a New Model No., all other changes would be handled through a "Production Change Order".
3. A Production Change Order may or may not trigger a New Serial Prefix. Typically, for instance, the change of a single component value would not cause a Serial Prefix change. I believe (but I'm not 100% on this), that would be decided by Production Engineer in conjunction with Product Support folks, who are responsible for Service Manuals. Major Engineering Changes would always trigger a New Prefix.
4. Instrument Manufacturing is done in Batches, called Production Runs. The Size of each Run is determined by Sales Volume. More Sales, Larger Runs.
5. A New Serial Prefix would be implemented for the Start of a New Production Run. Before issuing Work Orders for a New Production Run, I believe a review of Engineering Change Level would be done and the decision to introduce a New Prefix would be made or had already been made. That could be due to one Major change or sometimes a roll-up of a bunch of Minor ones.
Keep in mind none of this is instantaneous, 'cos of lead times for (new)material to be provisioned.
6. In an Emergency (quite rare), it has been known for a Production Change to be implemented immediately, maybe 'cos of a Safety concern(I did it only once) or some major Defect being discovered.
7. I think by now it has been well established that the Serial Prefix first two digits are the number of Years since 1960 and the last two are the week number of the last Production change included in the Instrument.
Sorry for being a bit Verbose, but that's how it works.
Paraphrasing my understanding of the above:
1: The model number and its letter suffix identifies the instrument type. Each is considered to be independent of the others for serial numbering purposes, and starts with a 00101 serial number suffix.
2: An expansion of (1) reiterating that the serial number resets with a new letter revision to an instrument when major changes are made. If it is only a rev change then the serial number will continue to increment sequentially from the last instrument made under the previous engineering rev.
3: Minor changes would not trigger a prefix update; more major ones (but those that were not enough to trigger a new model suffix revision) would trigger a prefix update.
4: When a design is released, they build a bunch at a time.
5: This one hung me up for a bit until I thought it through a bit more and paid attention to the wording. A new serial prefix would be implemented at the start of a new production run (note that it's 'would be', not 'will be' or 'is' - it only changes if the design is revised) Prior to the new run, they'd look everything over, and determine if a new prefix was needed; if it was not then the new run would get the old (but current to the design, as it's essentially unchanged) prefix.
To summarize my understanding of what 2-5 mean, for example, if the last unit of the run of the mythical HP 0000A Whatsis (introduced in the 32nd week of 1972) serial number 1232A00250 is finished and the design undergoes a major change to become an HP 0000B Whatsis in the second week of January, 1975, then the first new HP 0000B off the line will be serial number 1502A00101 (new prefix, reset serial). If, instead, the new iteration of the HP 0000A comes off the assembly line with a lesser engineering change (not enough to warrant it becoming a 'B' model), then the first unit in the new run will have a serial number of 1502A00251 (new prefix, incremented serial). If it's just a new run with no real changes, then the first off the line in January of 1975 would be serial number 1232A00251 (old prefix, incremented serial).
6 & 7: self explanatory.
I left the OP's name off because it was posted to the mailing list; if anyone wants to dig it was posted on 10/05/17.
Hope this clarifies things.
-Pat
<edit - added missing leading zeroes to my example serial numbers.>
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