The 300596 number is the main identifier for the Jeep-spec Dana 20. The -45 and -125 on the end are just Dana’s internal batch/revision codes and don’t indicate any functional difference. Both of your cases are the same Jeep-pattern D20 and both will work with our master rebuild kit here.
Dana 20s do have different helical-cut gear sets across early/late production and between Jeep vs. IH units, but those differences are determined by the actual gears inside the case, not the suffix on the tag.
If you’re deciding which one to rebuild, just go with the case that’s in the best physical condition (clean intermediate bore, no cracks around the front retainer area, good gear condition). The tag suffix doesn’t affect compatibility or gearing.
