It’s a jiggle pin intended to prevent anything from blocking the vent. In operation, the pin jiggles knocking free any trash and preventing oil that weeps from caking up and blocking. When your seal on the output shaft from the transmission fails, this is where you will see the oil weeping. In other words, it’s never a good thing to see fluid here. Dozens have reported this failure on this site, the reason I was easily able to diagnose my L5740 when it failed and the reason I now have the L6060 - which is prone to the same failure mode but the dealer made me a deal I couldn’t refuse when I brought my failed L5740 into the shop nearly 3 years ago. I’m now getting close to the same hours on the L6060. By the way the leak happens to those who use the tractor hard and mine works hardest in the winter clearing snow when fwd is always engaged.