These are the numbers you want to search for: 463981-001 538042-001
The Spares P/N will not get you as many hits on eBay as the Assembly P/N because the number on the side of the heatsink generally is the Assembly one. Sometimes both will be there. Same exact heatsink.
Look at the lower capacity heatsinks for the Z400, number of heat tubes (2 versus 3) and the motherboard plug end wiring, and compare those two things with the Performance heatsink. There are several revisions for the Performance heatsink.... any of those will do fine. Some sellers sell a Z600 heatsink as a Z400.... study eBay pics for the Z400 ones to really figure things out, and look at the part numbers. It's not as if one can't fit on another workstation but I'd stick to the official numbers to make sure there are no unforeseen issues that physically might make things not fit perfectly.
You won't ever find those specs from HP...... waste of time. Once you get one of these you can unscrew the fan and look at the fan label to figure out what HP used. Lots of the fans are identical across different heatsinks. Many have a HP P/N on that label, and the OEM source name and even OEM part number which you can use to find the OEM specs. Some have been modified to HP engineering needs, however, and so if I'm going to swap in another fan I look for the HP part number on the fan label.
The plug end wiring has to do with how the motherboard knows a Performance heatsink is attached. You can still find these both new and used off eBay easily, and elsewhere. The heatsink never wears out. The fan hardly ever wears out. Don't bother with trying to retrofit some other heatsink from another vendor into that, in my opinion...... you know the HP official one works properly, and you don't want to risk your workstation. That little black ground jumper wire from pin 1 to pin 5 is key to the motherboard knowing the proper Performance heatsink is attached to cover the heat output from your new processor. The pin order is otherwise conventional.... 1=ground, 2=12vDC, 3=motor rpm sense back to the motherboard, 4=PWM control from motherboard out to the motor.....
