Thanks to everyone for the suggestions. As I said, those EPROMs were in the machine when I got it and worked with the older TOS. I have to assume they're rated fast enough, but I can't actually find any information about NEC D27256D and don't know what speed they are actually rated for. I have anoth...

Attached. TOS104US.{LO0/LO1/LO2/HI0/HI1/HI2}. Original 192K image from tos104us.zip from the above site. Those ones were split with ROMSPLIT on Steem and verified as identical to the burned ROMs.

I have been trying to upgrade my 520ST from TOS 1.02 to 1.04. It had six NEC D27256D EPROMs in it containing the old TOS and were working fine. I erased said EPROMs in a real EPROM eraser (none of this leaving them in the sun business) and then programmed them with a QG-4X programmer which I've used...