The PROM chips originally shipped from Signetics, blank, having all outputs at logical LOW. L. J. Scully would have to program them by applying a high voltage to certain pins which would fuse a Ni-Cr (nickel-chromium) link internally that would convert the given output to a logical HIGH state. By using the DATA I/0 model 29B, I was able to clone the four unique PROMs from the Eva - Tone LS-76 (#660) (used as master devices) to four NOS N82S126 PROMs (target blank devices) so that the Display Processor board from the Nigerian LS-76 (#662) would have working PROMs and so that there would be inventory for #660 and #656 in the future.
If you have an LS-76 and need PROMs, we can help. We have more blank PROMs and the fuse-link process with the vintage 29B is confirmed to be successful (as of 18-2-2020), when we tested its results in the Display Processor board of #660 (shown, below).