The 5831 is later than the 5531, and that is later than the 41M (the 41M is old, the 5831 is newest).
For best road speed a 5831 with the 0.73 OD is your best choice. The difference between that and 0.85 is significant.
You won't notice the difference between 0.83 and 0.85.
I have a fair amount of info on the newer Spicer auxiliaries (5831, 6xxx, 7xxx, 8xxx, and 1241), some on the small Fuller 2 speeds, but none on the Watson or Brown Lipe.
I have also been trying to find out in what years each of these companies merged or were bought by Spicer, in the end Spicer seemed to have absorbed all the auxiliary manufacturers (except Eaton/Fuller).
Grigg
