Hey Casey,
Thanks for looking into this. I've only mounted one EFI Partition at a time. I have four physical disks in the system. I've attached a diskutil list. disk0 is the Windows NVMe disk. disk1 is the Apple NVMe disk formatted APFS. I've mounted and checked the EFI config.plist and it is set as 20,2. disk2 are the APFS Volumes for the disk1. disk3 is the hard drive for the Catalina Backup formatted APFS. disk4 are the APFS Volumes for the disk3. disk5 is a hard drive for my Windows Backup.
I'm using MountEFI to mount the EFI Partition. I've mounted "Catalina Backup" and config.plist shows iMac19,1 which is correct. I've not changed that. I've mounted "Catalina" and config.plist shows iMac20,2 which is what I changed it to with different SN, etc. "Catalina Backup" has Catalina 10.15.6. "Catalina" has Catalina 10.15.7. I haven't made a backup since I updated "Catalina". I've selected both from the Picker and I check and am booted into the correct version of the OS.
One thing I notice is the MountEFI partition that is listed for "Catalina" is disk2s5, however the actual EFI Partition is disk1s1. Same is true for the "Catalina Backup". It is shown in MountEFI as disk4s2 but the actual EFI Partition is disk3s1. I've even selected MountEFI option to mount the EFI Partition of the Boot disk. The config file shows iMac 20,2.
When I reset CMOS, I did reload the BIOS settings from a saved file. I'm not aware of BIOS containing this machine information, but I did not reconfigure BIOS. The only other thing I notice is in the OC Picker, there is a boot drive named EFI. Don't know where that is coming from and it is not bootable.
As far as I can tell, "Catalina" EFI is correct and I'm booting from the correct disk. I've also done an fgrep on the config.plist file to look for the old SN but it is not in that file, it is only in the EFI Partition of "Catalina Backup". Don't know where Catalina OS is getting the SN, etc when booting from "Catalina". Everything seems correct in OC. I don't want to start over wiping the disk now. If I have to do that for Big Sur, then that would be the time.
All with OC 0.6.1.
I've attached diskutil and MountEFI examples.
Thanks
Rand