I have gotten an almost complete set of Triumph Pre Unit Motorcycle Technical Service Bullitens from 1951 - 1959. Very interesting reading - I think. Anyway TSB #93,June 23 1952 explains why Positive Ground was decided upon. Check it out here in my Virtual VinMoto Garage (CP's)in the TSB folder!
https://www.vinmoto.org/gallery/v/VinMoto-Garage/CO-OHVinMoto-Garages/
https://www.vinmoto.org/gallery/v/VinMoto-Garage/CO-OHVinMoto-Garages/