Should also mention that I can usually tell an early 26 installation if it hasn't been upgraded.

Early installations used a KM vent valve instead of a No 8; P2 instead of a P2A; a pair of relayairs instead of an A-1 charging cut off; and an MU2A with an F-1 selector was common.

The P2, 26C control valve and 26F quick release all had design/operational flaws. The P2A replaced the P2 and I believe most systems were changed. The No 8 or a KM2 commonly has been retrofitted to replace the ring and piston KM and its COTS time restrictions. The 26C and QR both has retaining checks added to replace straight vents, subsequent castings integrated them. The F-1 selector allowed a 26LU unit to lead or trail (at least brake wise) any earlier one-pipe or two-pipe system. The one-pipe (6 & 14) systems have largely disappeared with time, and the F-1 has either been removed or blanked off.

The conversion to composition brake shoes caused some problems. The flatter friction curve made the wheels less likely to slide at low speed, but also didn't give enough static power on grades when stopped. The initial idea to correct this was a version of the SA-26 independent brake valve which had a "parking notch" in full application that substantially increased the brake cylinder pressure. As long as pure sets of like models were operated, this worked fine. After they started mixing and matching power (and brake arrangements), especially after the mergers, this likely caused a lot of slid wheels and there was another change. The use of double or triple diaphragm relay valves to make it a unit (rather than mu) function to adjust the brake cylinder proportionally based on whether it was single shoe or clasp arrangement.

26NL was a common conversion of 6, and new installations (especially on switchers). This quickly fell out of favor when the FRA failed to include the 6N distributing valve in the 3 year COTS exemption that 26 otherwise enjoyed.

26L with a 26D control valve was used on some switchers. As so few locomotives use the graduated release feature of the 26F/26C, the 26D should have become standard except they failed to include brake cylinder pressure limiting in it. 27L appears to be fairly rare in North America, seemingly limited to MOW equipment and trackmobiles. There is a weird combination common in export that for some reason uses a 26C automatic with the SA9 independent (from the 27L).