Happy to help......
Z400.... Revision 1 has 6 instead of 4 memory slots, and a later Boot Block Date. I have posted recently on the fastest unlisted dual-QPI hexacore processor that runs in the Z400 Revision 1 (X5690) and a cheaper virtually-as-fast single-QPI hexacore processor (W3690, sSpec code SLBW2) that also runs and is listed in the latest v54 3/8/13 HP QuickSpecs for the later Revision 1 Z400s. Both run hot at 130W max TDP, but most of the Z400s I have seen come with the bigger higher capacity "Performance" heatsink/fan. It has 92x92x25mm fan versus 80x80.
The more expensive processor can, run also in a Z600 Revision 1 two-processor workstation (because it has dual-QPIs) but has been proven to run also in the single processor Z400 revision workstation. I don't know if a W3690 could run in a Z600 as a single processor, but I'd not be surprised. Some Z620s come with one processor and HP warns you that you can't add a second.
This is one rare example of a processor that was never listed in the Revision 1 HP Z400 QuickSpecs that actually runs just fine. A project from a couple of weeks ago...... The QuickSpecs never, ever, have been clear what version they are referring to. You have to figure that out yourself, or with help from friends or friendly HP engineers like Dan_in_WGBU.
It costs about 220.00 USD versus 150.00 for the dual-QPI versus the single-QPI processor versions, so that was not a cheap experiment, but now you know.