It says 25 9 61. I'm guessing that's September 25, 1961. (It's just a year younger than me.) And just to the left of the numbers it says "ESM".
Here's the ball clamp next to the can of "Rust Fix" I used on it before putting on the primer and the finish coat. I picked up the rust stuff at Schuck's automotive store.
Here's the tongue minus the ball clamp that was bolted in place. The rust came off pretty easily with naval jelly, and then I hit it with a wire brush on the end of a power drill.