What PCB version and rev do you have, and which printing of the schematics does it match?
Note the PCB version and rev are two pieces of info. The version is a dash plus a two-digit number, usually written in sharpie somewhere on the board, usually on the parts side, near where it has the PCB part number. It will be something like -01, -02, -03, etc.
The PCB layout rev is a capital letter in a square, roughly 1/2" square, printed on the PCB somewhere. It should read A, B, C, or D, etc.
You should still very carefully check to make sure the 555 circuit matches the correct schematics, for all pin connections (i.e., get the right schematics, and make sure all pins match the expected connections, with a DMM, not just visually).
The 2055 EAROMs used for score saving on all of these Atari boards are the same and all need roughly -28V to program. It can be a little higher or lower, but -26 is on the low end. The difference could be due to other components, but I can't say for certain, as I haven't actually measured many of these Centipedes, and can't say if the schematic changes matter either. (I know on vector PCBs, it's always -27 to -29V, unless something is wrong).