That is the oil level check plug for the transmission and final drive. The oil should be to that plug when the tractor is setting level. Food for thought on your final drive oil problem. The hydraulic is not hard to remove. It might be smart to remove the unit and then look into the final drive. If you do this it might be wise to take the pan off the hydraulic unit too when it is off and make sure it is clean. I see you 55 is not one of the first ones but it still is 45 year old or older so a good look might be in order.