So, when was the 26th founding? This is very important for modeling the Mentors. Lexicanium and 40K wiki say 738.M41. However, the Mentor Legion WD article mentions they were founded in the middle part of M41. I wouldn't consider 738 exactly middle.
The thing is, if they were founded 738, they are very fresh and that would have to be taken into account - they would be very early in their learning process. Otoh, Honour The Chapter has a Mentors item that is centuries old.
So, which is correct?
Alex