I've been poking around a bit and it appears the BA 10/5 came with two different ratios for 5th.
Input : 26 teeth, Layshaft Input : 37 teeth, Layshaft 5th : 53 teeth, Output 5th : 27 teeth, Ratio : 0.72496
Input : 26 teeth, Layshaft Input : 37 teeth, Layshaft 5th : 51 teeth, Output 5th : 28 teeth, Ratio : 0.78129
:dunno: