One thing about buying a 60 year old trailer is that a lot of the original information about the trailer has likely been lost. Previous owners will tell you the tires are great, there's no rot, "everything works" and it's camp ready. In our case, the trailer came with a small receiver hitch with a 1-7/8" ball which we assumed was correct.
For our first little tow, we borrowed a 1-7/8" hitch from a friend and everything seemed fine. It's hard to tell if there's a problem when the trailer is on the ball, but our trip was beset with all manner of clanks, bangs and clunks. To find out once and for all, we purchased 2 balls, 1-7/8" and 2". As you can see below, the 1-7/8" is a bit sloppy.
The 2" ball fits pretty well and the hitch latch closes completely, so I think we have a winner. For anyone out there with a 60s Hadco MK-61 coupler, make sure you try the 2" ball before taking a long trip.