I hate to contradict jdemaris because he is usually absolutely correct, but I think he is mistaken on the plastic tube. That is strictly a vent for the reservoir.
That is not universally true. On the Dubuque/Manheim 1020, 1030, 300, 301, 302, 300B, etc. . . . Deere always used some sort of system to bleed up oil from the bottom of the power steering valves - when it's a closed-center system. Some early machines have open-center steerring a differ a bit.
The way Deere did it varys with machine and serial #. Also varies by option package- with hi low, with reverser, without either, etc. I have two 300s and one 300B and all three have plastic bleed-off lines hooked to the bottom of the closed-center steering valve. Deere used to do the same with the front hydraulic pumps when 2.4 c.i. or bigger.
The little hydraulic reserve tank in front of the radiator has several lines hooked to it, along with a check-ball. It serves several purposes, NOT just one.
I cannot say one way or the other what this person has on his machine. I havebn't seen any photos and often on machines that are 40-plus years old, things get changed over time.