This might be a silly suggestion, but there might be confusion among sources, about the obverse or reverse side of the flag. I wonder if the conflicting descriptions come from having confused one side with the other. If the obverse side (ie, viewed with the staff at the left) showed the first canton (ie, upper left) as black, then if we looked at the other side of that flag, the upper right canton would be black. Now, confuse the obverse and reverse by moving the staff, and now we have 1st canton red, 2nd canton black.
Still, it looks like 2 votes to 1 for black in the 1st canton, in those sources.
BTW, I was mistaken in my memory about the Osprey series. There is only a general explanation of the system of colors that French regiments carried, and the design, but nothing more specific about the individual regiment's colors.
Prost!
Brad