Everything Brendon says.
In general the same Planet Jr has been around since the late 1800s. In the 40s & 50s, JD,AC, IH and others all used the Planet Jr for vegetable planters.
To get the best seed distribution, you bench calibrate. Place seed in the seed box and turn the drive wheel to correlate with a specific distance then count the number of seeds dropped. If 60 seeds drop in a 60" distance, then average seed spacing is about 1". If 30 seeds dropped, average seed spacing is about 2".
Another factor is seed size. Seed drop can be more consistent if seeds are sorted by size through a seed sorter.