http://www.psacard.com/pop/t206/Index?byback=1&pf=0
Not sure when they revamped their Pop Report pages, but here's the T206 PSA link above...
Am I missing something, or did they eliminate the "Unknown" back pops in this newer chart? I do not think the numbers add up. Take T206 "total population" graded and see individual back numbers below...big gap! the balance is Unknown, before they started breaking them down by back brand...