[dexter1]

I would like to add another $0.02

But I think you (and the posters in that old comp.sys.sgi.admin thread) have gotten rather far out into the weeds.  As I would seem to have found, it's a pretty simple problem to solve:  Stuff a blank E^2 into the socket.  Power the system up, which initializes the E^2.  Use the monitor (3K) or burner (3K, 4K) to enter the MAC address of your choice.

If it is a blank 93CS56/EEPROM/FLASH/NVRAM/E^2/DIL then i would expect the Indigo HP1(R3K) and HP2(R4K) board to behave differently when it comes to initializing the chip as opposed to backplanes already having a chip initialized. Have you checked all the different combinations? If i missed this in your previous post, apologies, i blame my geriatric eyes.

I found elsewhere discussions of how the 3K ROM monitor - but not the 4K - can be used to change the address, and that's fine, but a fussy difference.

I do not seem to have the procedure written down for reprogramming the empty EEPROM with the R3K monitor. Can you post that procedure, or point us towards that info? For EEPROM's already initialized, Dave Olson (that quoted '93 post from the thread i linked) mentioned that networking of a HP1 in an R4K initialized backplane doesn't work which i reproduced in my HOWTO, since after all, Dave Olson is Dave Olson.

No fussing about with protection registers or checksums needed, afaIk.

Regarding the checksum, that doesn't seem to be necessary when programming it, it will still work. However the EEPROM protection register needs to be reset when preparing an already initialized R4K backplane EEPROM for accepting an HP1. The protection register is not permanently locked however, they do not seem to bother perma-locking it in the Indigo systems.

One of my fellow countryman SGI collector had this exact same issue and i pointed him to my Nekochan post. Alas i don't know if he was successful in reviving the network on that system...

Ideally, it would be best to keep the pair of board and backplane/case together, but after 30 years...