Hey Ron, here's what I could find:
Lee makes a 515-450 and a 515-500
Both nice looking slugs!
RCBS makes a .520 #82170
Lyman makes (or made) a 515-139, 515-141 and 515-142
Plus a 518-145
I'm also interested in this topic. Really liking the Knight 10% discount on their 52s but I would want a mould to cast my own full sizes in case the supply dries up (plus it's just plain fun to diy)
btw: this topic finally forced me to member up here!