When I bought my 20' Nash about 21 years ago, I also picked up this coupler lock made by Fulton. I see the current locks have round keys, my antique lock has the normal straight key.
During the past 2 decades it weathered well and the lock/key mechanism never jammed. Does it work? I still have the trailer. When I lived in the "void" south of Oregon and north of Mexico, about 2x a month someone would knock on the door asking if my Nash was for sale. In our quiet, well to do neighborhood, travel trailer thefts were not unheard of.
https://www.fultonperformance.com/product/63227_trailer-coupler-lock
