My old trailer and the new one have that type.
A bar holds the ball from the back. The front has a perminant lip that holds the other side.
Rust and general crud, possibly a coat of paint, are between the bail and the front of the coupler. Clean it. Penetrating oil. Work it up and down. And back and forth until it frees up.
Once it is operating smoothly. Oil the bail and the slide at least 4 times a year.
TT 2015 19RD "TheJayco"
TV 2003 F-350 "Montblanc" - Housebroken chore truck
Sitting in The Cheap Seats.
And proud of it!