****first thing you do, before you try using this old plow,******
attach a saftey chain from the plow to the tractor around the snap couple area.
if the snap couple comes dis-connected while you are plowing, the nose of the plow will go down into the ground and the rear of the plow will come up and flip over on you, before you know whats happening.
the other (snap shut) type upper arm attachments were suspose to open and disconnet if that ever happened. But, i do not know if i would trust them with my life.
the safety chain would be your best bet.
good luck and be safe, that is a very old plow you are using, all snap couple equipment i ever had used the snap-shut type locks on the upper arms and they would come open, if in good repair.
any type hitch tractor can be dangerous,
my friend broke a lower arm on a 3 pt hitch tractor, he looked around when he heard the noise, but by then the bushhog was climbing up the other side tire after him, he got it stopped.
Ruined the tire and bent the other lift arm and top link.
was a mess, but he did not get hurt.