Yes, there are two versions.... the original "Version 1" and the "Version 2". The original can run the Sandy Bridge Xeon processors that HP certified for it. The Version 2 can run both those Sandy Bridge and the newer Ivy Bridge "v2" processors. A nice little article on the differences between those two generations of processors is linked below. Again, assuming you have a later version Z420 you generally can only the Sandy and Ivy Brige processors listed by HP can due to built-in BIOS microcode limitations (there are a few rare exceptions):
A trick that works for most HP workstations is to search, for example, for "Z420 Spare Parts" in Google.
At the top of the results you'll see the HP page for that, and note under the Z420's Processors section that there are 700000 series part number processors with IB (Ivy Bridge) in front as available processors. Note also down at the Miscellaneous section at the bottom are a couple of 700000 part number motherboards. Those are the later Version 2 ones. The -001 comes pre-set for W7 installs; the higher number for W8.
I don't have the BIOS boot block date for easy differentiation between Version 1 versus Version 2 for the Z420, but that is the best way to tell if you have one. The boot block dates for the Z620 and Z820 are 12/28/11 for the original and 3/6/13 for the Version 2 motherboards. Look in your BIOS under the first tab for that.... Your Z420 may not have those exact dates but should be close, and you'll know from that.
Thus, if you focus on QuickSpecs and HP literature from the early 2013 era you'll start seeing the Ivy Bridge processors being referred to.
You can look at the Intel "Ark" site under the HP approved v2 processors to see if there is only 1 sSpec code, and if so you can be sure that you are buying an identical used eBay processor to upgrade to if you go that route. Search eBay under the sSpec code once you find that at the ark site.... it will be linked at bottom left of the front page for each processor you search for.
I've upgraded processors into the Z400/Z600 workstations but not yet a ZX20 workstation. Same process I am sure, just more expensive.
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