This rather homely, baseball-only strip card set contains 20 players and slotted the incomparable Babe Ruth fifth. W521 closely parallels one of 1920's W519 strip card sets, copying--but reversing--its images and numbering system. You can easily distinguish one set from another by the player's shirt, which buttons (correctly) left-over-right on W519s and (incorrectly) right-over-left on W521s.
Based on periodic searching, strip cards markedly lag more "popular" 20s and 30s sets in available information. I finally found a decent W521 checklist at subscription site Vintage Card Prices. It includes a scan of #15 Ray Schalk, a White Sox backstop selected to the HOF on the strength of his excellent defense (and penchant for catching no-hitters).
Value: A 2006 auction yielded this W521 Bambino for about $125. My version features heavy wear to the corners and edges, but could be much worse.
Fakes / reprints: I particularly recommend using personally trusted dealers for cards of Ruth, since he generated more reprinting and counterfeiting than any other player.