M20-2 is one of the two normal fine pitch threads for metric, used in Europe and UK. The other fine pitch is 1.5 which I think is more common in Japan, China etc
Some dingbat in CIH thought it would be cool to spec M20-2.0 for the holes under the transmission housing.
Actually, this tractor hasn't started the season yet. The clutch took a dump last winter and I have finally started on it. Will need it in 6 weeks or so.