I just measured a dozen of the tumbled balls. Almost every one was .576 - very consistent - with two or three very slightly larger - maybe .5765.

Now I really wish I had some untumbled ones to measure. Would they be closer to .575 (as the mold box states)? The tumbling leaves a textured surface on the ball, rather than the smooth surface left by the mold. So I'm wondering if tumbling increases diameter slightly - as with knurling.