If it were me, I'd just use the 50mm ball and not worry about it. Adjust the jaw for a snug fit with the lever down (or up, depending on the coupler). U-Haul rental trailers fit both 1-7/8" and 2" hitch balls, since the jaw is manually screwed snug. But all couplers that I'm familiar with have an adjustment on the moving jaw, and 32 thou isn't going to make any real difference if you adjust the coupler for a snug fit.
It's not like those couplers are precision machined to start with - I can see wear marks on just three places on my chrome hitch ball from the two points of contact in the upper-front (where the formed part touches) and the lower-rear (where the movable jaw contacts it underneath). The hitch would have to lift off the ball to get loose, and if the jaw is adjusted properly, it's not going to be able to do that even with a (very) slightly smaller ball.
But if you can find a real 2" locally, or bring one with you when you fly there, assuming you're in North America when not sailing, then you could always do that.
Oh, and if it's a US trailer, it's always a 2" ball. The only other size near that is 1-7/8", and that doesn't have the capacity at 2000 lb (typically). or 2-5/16" diameter for really heavy trailers, like portable buildings and very heavy boat trailers and big bumper-pull RV trailers (like 12,000 lb and higher).