As long as the OS wasn't reinstalled you can still see devices in Device Manager that are no longer attached to the system by enabling an environment variable (set devmgr_show_nonpresent_devices = 1), then enable view hidden devices. Once you do this look at the properties of that ghosted out proc, then on one of the tabs (details?) See if it lists any proc specific identifiers like a serial number or CPUID. This will give you something at least. Best would be something like a receipt of purchase, but the issue is not if you have, or had, or ever purchased the processor, but rather proving that the specific proc was in fact in the computer when you turned the device over to them.