I would think that most people who would spec a four and a quarter cat would go with the engine brake option BUT each build is different but the above poster pointed you in the right direction, look behind the panel.
Remember that corporate trucks are ordered after a bunch of negotiation between carrier and builder.
Often the corporate bean counters get their input and limit the actual end result. Just because you find the connections; the opposite end may not be connected and almost 40 years out of the factory can compound what is there or not and if it works or not.
