I've seen barrels as much as 0.3" difference that were stated as the same length.
All the measurements that I posted were on a closed bolt and installed extensions. An 11.8" barrel would give you the 26" OAL, but to be safe, a 12" barrel is suggested. I see why the manufacturers use the 12.5".
Yes that is measured to the end of the threads but there is a caveat, i based that off a 10.5 measurement, which is exactly 24.425. I'm assuming when it comes to barrel length the measurements are on point given the nature of the ATF rules.