Not sure if you're joking, but that's a MacPro4,1 flashed to 5,1 with SMBIOS spoofing to run Big Sur. That 2.7GHz Core i3 is actually a 2.66GHz Xeon W3520.
Hmmmm..., when Big Mac ran a serial number check, Apple responded with invalid serial number. Does flashing with SMBIOS spoofing invalidate the machine's serial number by erasing it from the SMC chip? Also, what does the SMC if anything have to do with a serial number?
Upon a little further research, it can be discovered that SMBIOS flashing has a "How to configure your system's SMBIOS correctly tutorial at tonymacx86.com/threads/guide-how-to-configure-your-systems-smbios-correctly.198155/, looks Big Mac nailed it, GSXB is running a Hackintosh rig. That is the only explanation for the invalid serial number response from Apple.