The Air Brake part numbers and brake schedules are simply a confusing nightmare.

The late Dave Garcia and I one night started naming all the valves that had been designated as A-1, and when Dave started adding in all the traction side it just got silly.

Incidentally, Wabco has an A-1 schedule for steam locomotives that included a G-6 for the automatic brake valve and a S-3A straight air independent brake valve. Some locos used a steam jam brake in place of the straight air, and NYAB had their own brake system.

This was replaced by the 5-ET schedule (ET = Engine Tender) while NYAB had the LT brake system (Lt = Loco tender). It was soon discovered that the 5-ET had some undesirable design issues, so it was quickly replaced by the 6-ET with most of the former 5 equipment returned and upgraded. There may have been a No. 7, it was common in Australia, but in the US it became best known as the 14-EL (Electric locomotive) which was a double ended set of No 7. There was a No 10, 12 and 14 series of distributing valves which were designed for double ended equipment, otherwise the stock 6 (or 5) series distributing valves were used.

6 series took a little detour with 6-BL (Branch Line) primarily by NYAB with Wabco providing the 6-DS (Diesel switcher) and 6-SL (switching locomotive) schedules.

There was a 8-ET for late large steam which also had a 8-EL for large electrics and early road diesels (such as the FT). The 8 evolved into the 24RL (Road locomotive). The first generation of 24 had an ET version, while the second generation included pressure maintaining against brake pipe leakage while earlier versions could be either be converted with factory parts or by railroad modification. I'm not sure how they managed to jump from 6, 8 and 14 all the way to 24.

In the 1950s, small switchers usually got 14EL (real small just had straight air), while EMD made 6BL standard with 24RL an extra cost option for locos intended for road service (though GMD used 6SL instead of 6BL). In reality, almost any common valve could be either NYAB or Wabco as they cooperated in the common designs to keep from getting into monopoly issues and has done so for decades. For instance, while 24RL was primarily Wabco, I think it was GN that got NYAB 24RL because they were a dedicated NYAB customer. GP&s and F7s could have either 6 or 24 depending on what the railroads ordered.

25 was only built as a single test set. An ill fated attempt to convert 24 from ring and piston to rubber diaphragm. It reportedly only made one actual test trip on either Pennsy or NYC.

26L actually originated from the transit side of Wabco and was used on some of the early articulated lightweight trains. The freight side of Wabco adapted it to regular heavy passenger and freight service around 1957 when it was fairly quickly adopted, though 6BL, 14EL and 24RL was still available. 26L initially started as numerous brake valves mounted to individual pipe brackets and everything connected by pipe or copper tubing. This evolved to many of the components under the cab being mounted to a panel which also had some of the internal connections and volumes mounted to it. There is also a 26-LU schedule (Universal) that included a F-1 selector valve to allow a 26-LU equipped engine to MU with any one pipe system (unmodified 6 or 14) or two pipe (24 and modified 6) as well as 26 (which is typically two pipe).

27 is a lighter and simpler design that saw some limited application in the US on small diesels and things like cranes and trackmobiles. It seems to have been much more common in export use, including a hybrid 28 system that combines the 26 automatic and 27 independent, and presumably the 27 control valve?

Not sure what 29 was supposed to be.

Schedule 30 was the "desktop" brake valve which is basically uses the 26 brake arrangement except for largely separating the engineer's brake handles from the valving.

As Dr. Z mentioned, 26-C was a control valve arrangement primarily for heavy rail passenger, while 26-F is for locomotives. They have different pipe brackets and some other differences, and 26-C has an emergency portion while 26-F has a quick release portion to allow the engine brakes to be "bailed off" after a brake pipe reduction. There was also a 26-D control valve which is a simplified 26-F but didn't allow for graduated release that only saw limited application primarily on switchers at least in North America. To add to the confusion, the 26 control valves have about 12 different versions depending on where they are used, but are visually identical. And the common engineer's brake valve is also designated 26-C. There is also a version of 30 only for passenger and one for freight that aren't compatible but look almost identical. There is also a 26-E and 30-E which are position brake valves apparently used for PS-68 braked cars. And, there are versions of 26 for passenger or freight that use common ABD series valves in place of loco design valves.

Schedule 31 was the first electronic controlled brake system from Wabco. The first design was 3101 which used "pneumacards" basically trying to make the system with plug in modules for easier troubleshooting and repair. The weight of the "cards" and inability to keep them from leaking at the plug in rack resulted in it quickly being replaced by the 3102 which returned to the more typical bolting of components to a panel.

I have no clue what the modern electronic control brake valves are designated as though there is obviously several generations and variations in design. Wabtec/Wabco and Knorr/NYAB have gone their separate ways, though I believe you can still mix and match the Knorr DB10/DB20 portions with Wabtec ABDX on freight cars per AAR standard. Funny thing - even the most modern electronic system still has a fully pneumatic "triple valve" to still apply the brakes if an engine is moved dead in train.