Are you asking about the cotter pin hanging from a hole in the bottom of the bell housing?
If so, that is the drain for the bell housing. It is there to let any oil that enters the bell housing drain out. The cotter key keeps dirt from clogging the hole.
Are you sure it is transmission oil? It can be transmission or engine oil from the rear main.
Check the transmission oil level and look for water contamination. If there is water in the transmission, or it is over full, it will leak past the front seal. If the transmission was recently filled through the plug beside the shifter, and the oil was added to quickly, it will leak past the front seal, then stop once the level recedes into the rear housings.
But sometimes the front seal is just bad and will need to be replaced. Not a hard job once the tractor is split. I believe it can be changed from the front, at least my TO35 was changeable from the front.