A quick look at Kirkland's book shows that the P12-42 indeed had a constant speed power plant, running at 850 RPM. The HP was divided with 1200 for traction, 400 for HEP and 150 for parasitic loads. The HEP alternator had an output of 233 kw, 436 volts. 56.7 cycles, which I imagine was close enough to allow the use of commercial 440/60/3 heating and AC equipment.
Interesting find, as I had always thought that the GE U-34CHs that I grew up with were the first units to use constant speed engines, with changes in the traction alternator excitation used for locomotive speed control. I guess they were the first major locomotive use of that system, at least-
Somewhere I have a book, with schematics, on D/E tugs with Allis- Chalmers propulsion. The engine ran at half speed most of the time, with slower speeds controlled by field excitation. Above half speed, the excitation was maxed out and the engine RPMs determined the shaft speed. This was in the late 1930s, and I suspect that the WWII subs had similar systems, with either F-M or GM/Cleveland power.
The BLW units had a separate Maybach for HEP, and the Aerotrains had a pair of 6-71s (GM, what else-) wedged into the nose, if I'm remembering what I've read correctly.