Here we'll show the cover of an ALCO-GE sales brochure originally printed February 1950 and reprinted August 1950; both dates appear on the rear.
As we can see, the model being advertised is the ALCO-GE 1600. No model number for this locomotive is used anywhere in this brochure.
The locomotive in the airbrushed view on the front cover is an Alton & Southern locomotive, which normally would be classed by railfans as an "RS-2" considering that its fuel tank is mounted underneath its cab, its battery box is under the frame, and the locomotive was built rated 1500 HP. However, according to ALCO-GE sales literature and operating manuals this properly was a "1500" and the cover illustration is clearly being reused from an earlier edition.
While most of the illustrations in this brochure show the belt-driven auxiliary generator and exciter commonly associated with the 1500, the illustration shown by the heading "Main Generator" shows end-mounted auxiliaries. Further, the speed-tractive effort curve in the brochure gives a continuous rating of 52,500 lbs, commonly associated with the 1600 or "RS-3."
Of course, all that the curve really assures is that it's accurate as of 8/50 and not that any of the illustrations inside would match. With careful analysis, then, it's pretty safe to say that the previous cutoff of model numbers, RS-2 vs. RS-3 correlates with the change in fuel tank location, change in main generator model, change in mounting style of aux. generator and exciter, and change in rated continuous tractive effort. The increase in rating of the engine to 1600 HP happened briefly before all these other items were altered.
All that aside, the illustration is still impressive!