I looked at this a while ago and found a remarkable variation in the equatorial measurements on balls stamped 1 7/8. Some were much closer to 2"; some were almost a full 1/8 under (i.e. closer to 1 3/4). I've heard stories of trailers coming off an undersized 1 7/8, but I don't know if the stories were backed up by actual incidents or just stories made up by active imaginations. Regardless, since 50mm balls are readily available, why not just pair them with 50mm couplers when you have one?