From roughly the 1930s to mid/late 1960s, most British constructors used positive earth, believing that it reduced corrosion on electrical terminals, and that it was kinder to spark plugs - in coil ignition systems - to have negative sparks. (So says my 1967 edition of the Penguin Car Handbook, by Robert Ireson.)
The latter is true enough, but is irrelevant with a magneto, where you get one pos and one neg spark on a twin, or two and two on a 4 cylinder engine, whether you like it or not.
By the 1970s all the manufacturers had pretty much gone back to Neg earth, which continental Europeans hadn't wavered from (and nor had Rolls Royce). Materials were better, so the 'issues' were now regarded as minimal.
Positive earth is often said to cause more galvanic corrosion to bodywork and so on, but every Brit car I ever had of either polarity from those years turned into ferrous oxide pretty darn fast . . . steel treatment or lack of was a big part of it, and paint wasn't up to much, as all those businesses selling underseal products found to their advantage.
For no really good reason, I have my bikes wired positive earth, but it's just habit really. Negative can be helpful for attaching modern electrical gizmos I believe, although I don't have any, and was helpful for LEDs initially as mentioned.
