Every once in a while a well meaning citizen moderator locks a thread in this HP-hosted but for-users-to-help-users forum when they see something about improving used HP workstations.
Of interest, there is actually a whole series of official HP web pages that provides such support. You can find one form of these by searching Google via "Z600 spare parts", for example. Let's say you had a HP Z600 version 1 early boot block date workstations with a motherboard that could not run the Xeon 5600 series processors that HP officially began support for only in the version 2 Z600 motherboards.
And lets say you were interested in finding the official HP part number for a replacement version 2 motherboard, and that you were willing to buy one of those and do the project of transplanting in this new motherboard. If you did that search on Google the top result would be the official HP web page, and there would be two motherboards listed near the top, and the description would clearly define which the older version 1 motherboard was, and which the newer version 2 motherboard was. As expected, the older version 1 has a lower number than the newer version 2:
System board (motherboard) - Intel Tylersburg-WS 2S platform, dual Tylersburg IOH, ICH10R controllers, 1333 MHz front side bus, 461439-001 (older, version 1)
System board (motherboard) - Intel Tylersburg-WS 1S platform, 1333 MHz front side bus (Rev. C2 chipset), 591184-001 (newer, version 2)
EDIT: If you do the same search in Google for "Z400 spare parts" you'll see the corresponding official HP page, and the version 2 motherboard description there also discusses the presence of 6 memory sockets on that later motherboard.