The 16V traces/lines of 3 of 7 S-100 slots (including the side ones) were burned out. I can get 8V on all slots however. I took the system apart and inspected the underside of the backplane to confirm. The only way to fix this is to put jumper wires to replace the broken traces. It looks like this system was done by hand, there is a lot of clean up necessary to get all S-100 lines on all slots cleaned up. ug.
UPDATE: All three Polys I have worked on have this problem. I have picked up some 28-ish gauge wire for repairs.