A spark plug can either "spit" the spark from center electrode to grounded tab, or "suck" the spark from grounded tab to center electrode. On these old engines spark is more efficient when it "spits" rather than "sucks".
This is controlled by coil wiring. Bob has it right that coil post to distributor should match system ground polarity as this makes the plug spit the spark. Reverse coil connection and plug will suck the spark.
Tractors that I have encountered where coil was wired backward they seemed to be more stubborn to start when cold, but didn't notice a big difference when running.