There is some conflicting information out there with respect to exact dates (e.g., the NMRA info looks suspect), but I tend to rely on the 1913 Westinghouse handbook for information about K brakes:
My understanding is that Westinghouse came up with the "quick-action" triple valve in 1887 (replacing the "plain" valves), and rapidly cornered the marked on air-brake equipment. The first such triple was the "H" valve (H-1, H-2), which was the superseded by the "K" design sometime around the turn of the century (I can't find the exact date with a quick search). These were just Westinghouse model designations, but they were the de facto industry standard, which is why you see "K-2 Triple" (or K-1) stenciled on so many pre-1927 cars.
In case the clinic didn't cover it, the K-1 valve was for use with 6- and 8-in diameter cylinders, while the K-2 was paired with 12-in cylinders. KC was short for "cylinder and reservoir Combined", and KD for "cylinder and reservoir Detached".
Hope this helps with the K side of your questions. I'll leave AB to someone who who models in that era.
Swall Meadows, CA