banging my head against … grub …. grrrr

So we had loaded two pretty darn nearly identical JackRabbitsландшафт for delivery to a customer tomorrow. As part of the load, we want serial consoles available in case we need emergency access. Plug it in and solve problems.
Remember, these are virtually identical machines. Same MB, same CPU, same rev (one has more cores/twice the memory of the other used for disk to disk backups). Same bios, same bios settings.
One would think they would behave the same. Right?