I'm going to stick my neck out and say that I can't see how it can be called a 'Negative Earth', as doesn't it just depend how it's wired. as per Johns' request??
Being barely adept at electrical generation, I'll consider this a test of my 'Knowledge is dangerous'..?
But serious,(well, more serious), and this is where I may be wronger, if you just switch either the brush wires, OR the field wires, should reverse the polarity(then join D+F to 'polarise' it)..?
How'm I doin'???
Trev'll sort you (and me)out if I didn't, but does it really matter if it's Positive or Negative earth??
Cheers, duTch.. I'm off to the sideline (or the sinbin)
Afterthought..... watch out for those pattern copy regs, the cover may not be high enough, and short out the Reg bobbin to the Cut-out bobbin , to 'D' , maybe to earth for good/bad measure....! Mine would have but I looked at it first.