My Lee 0.600 RB mold actually throws a 0.604 ball. I don't know if that is typical, but at $20 for the mold, it's worth a try.
I have seen this oversize casting a number of times with both Lee and Lyman moulds - and with pure lead, not alloyed. Taylor's .600" Lee DC casts .605" and .608" respectively. His Lyman .610" casts .613 or .614".
If you order a Tanner mould, order it .001" or .003" larger depending on the size you want, than the ball you need. Jeff cuts the blocks (rounds?) to the exact size you order. Due to normal lead shrinkage as it cools, a mould in the larger sizes will cast about .002" smaller than the hole size and one in the .677 Tanner (15 bore) size will cast .675" as does mine. My .745" Tanner mold casts at .740 - that should give you an idea.
If wanting a mold in the .605" range, I'd order a Tanner in .606" to .607". If it is for a 20 bore or .62" smooth, I'd order a .600" Lee DC or SC and be happy, just adjusting the patch as needed.
If for a rifle, I'd order the Tanner mould at .004" smaller than the bore size (casting .005" to .0055") smaller) and use the 10ounce denim patch.