How far back did LOCOTROL go? I didn't think DP was super common before the 90's. Could some of those SP trains back in the 70's have been manned helpers? I know BN used manned helpers back in the day in my state. They'd hook up at Balmer Yard in Seattle to push a train over Stevens Pass (Scenic Subdivision). MRL and BNSF still use manned helpers.
There are other threads here that talk about this, but the super brief history is that it started out in the 1960s and was typically installed in an unpowered Locotrol car connected to the Engines.
It was then placed in Locomotives, most famously the 1970s era "Snoot nose" SD40(T)-2 series of ATSF, SP, UP. Basically the 123" nose contained the locotrol equipment.
What changed in the late 90s is that GETS (GE Transportation) bought Harris Controls which owned Locotrol at the time.
DPU is a genericized name. Locotrol is a brand name.