I just don't know why. It works its way loose no matter how hard I turn it clockwise to lock. Maybe it's old. I can't find a cheap replacement part either. I'd love to switch to a latch coupler, but not a welder. Not sure.
they make bolt on couplers. just use good grade 8 bolts..

What do I need to do if I want to replace the old coupler with the colt on latch type?
I assume I need some type of hand saw? What type? To cut off old piece.
Drill bit- what size?
Bolts, bits and washers- locking washers? What size?
Sounds like it may need to be done in a shop!?