Some of the 514 checklists include a and b variations for cards with basically the same front image but with different teams or captions. They are
Doyle NY & Doyle NY NATL
Magee Portrait & Magie Port
Demmitt NY & SL
O'Hara NY & SL
F Smith White Cap Chi & (Chi & Bos)
Lundgren KC & Chi
Kleinow Catching NY & Bos
Elberfeld Port NY & Wash
G Brown(e) Was & Chi
Dahlen Bos & Bro
Peter's explanation of 518 and 520 are of course correct.