deciphering old serial numbers for mfr date

[Chas7]

I have a quartet of old ProCurve 10/100 12-port hubs that have to be decommissioned.  The catch is that the agency processing decommissioning requires purchase dates. I hope to appease them by using manufacturing dates. Serial numbers for these all begin sg9xxxxxxx. I know that the conventional wisdom is that the date is coded from characters 4-6 (year and week) but I am not certain. The other thing is, I cannot be sure that these dates are in the 1990s or 2000s.  Any thoughts? Thanks, C.

[parnassus]

Probably late 1990s or early 2000s.

20 Years old hubs and purchase date is really required?

You have really Museum pieces

DoM isn't strictly related to purchase date (not sure if that bit of information is necessarily coded into Serial Number into 4th, 5th and 6th positions as Y Year and WW Week)...it's clear that purchase date can be successive to DoM...the only fixed dates other than DoM are potentially (a) date of introduction to market and (b) device EoL date..of both the really significant one would be the (a) only since looking at (b) one can purchase a device well after it was declared EoL but can't do that before it was first introduced.

[Vince-Whirlwind]

No chance of finding any old IT Procurement documents? Annual IT expense documents?

Personally, if I had no access to those old documents, I would just make it up.