I can answer your question as best I recall, I read this somewhere LONG ago and Ive slept since, so NO WARRANTY:
Long BEFORE the Neg ground standard, "some" companies believed there was less galvanic action (dissimilar metal junctions can form a battery) and corrosion where frame members joined if you use Positive ground. Chrysler and Ford thought one way (Pos) GM I believe another (Neg).
Of course, strictly electrically speaking, current flow is still current flow REGARDLESS which battery post happens to be connected to some huge chunk of iron (tractor frame). Theyre simply using the tractors conductive metal frame as a current carrier so you can frame ground the lights and starter and NOT have to run wires to all those !!!!!!!! And the battery dont care which post you connect the hunk of iron to either, the frame acts as (replaces) the wire to carry current is all
This is NOT related to your question about Pos or Neg of the battery connected to the hunk of iron HOWEVER as far as current, electrons are negatively charged particles which flow readily in atoms with plenty of free excess electrons in their outer shell/orbits (like say Copper) and while the electrons flow from atom to atom (electron current) the empty vacant holes/spaces left behind since electrons got knocked out of their orbits (hole current) is in the opposite direction. Therefore electron flow is one way while hole flow is the opposite. At Purdue wayyyyyy back in the sixties we were taught conventional electron current flowed one way while hole current flowed the opposite.
STILL remember electrons are NEGatively charged particles and those electrons flow one way while the empty spaces/holes flow the other...
AGAIN NO WARRANTY THAT WAS WELL OVER 40 YEARS AGO I MAY HAVE FORGOTTEN LOL
John T BSEE Purdue late sixties