I have collected a few discs during the last couple years. From the number that I have there seems to be four sizes in the ball and socket type. Shown here is a 6 inch disc with a 7/8 inch ball compared to a 16 3/4" inch disc with the 1 3/4" ball.

Most suspensions under 10 inch dia used in N.A. are of the clevis type. In Europe they are often a 7/8 inch ball and socket.

The common 10 inch discs used in N.A. are most often a 1 3/8 inch ball and socket. Europe and in particular Australia seem to prefer the clevis type.

Many of the discs larger than 10 inch use a 1 3/4 inch ball. A few of the 10 inch also have the larger ball, as well many of the larger discs use the 1 3/8 ball.

I have one disc, EIV which is a Sediver product that has a 2 inch ball and socket.

A generalization would be that the small size is used in Europe, the common 10 inch disc uses the normal 1 3/8 ball, discs larger than 10 inch use the 1 3/4 " ball, and the 2 inch is scarce, likely only found on a few of the bigger discs.