This is a tip from Vlad. The Library of Congress has put a passel of Branch Rickey’s papers, including a lot of scouting reports, online. There’s a good bit from his time as GM of the Pirates. I haven’t gone through it to a great extent yet -- the site is pretty slow -- but I did manage to find some reports on Bill Mazeroski. They’re images 61-63 of the 1956 scouting reports A-P. Seems the Pirates liked Maz quite a bit.
Wanna see Branch Rickey’s scouting reports?