Mainstream, hah! Got the downpayment on a nice condo on Maui there, Peter.
1906 Winthrop Moving Picture Post Card of George Dixon vs. Chester Leon. Surely one of the most creative PC formats ever, it is essentially a flip book. Winthrops are known with baseball subjects but this is the only known boxer and only known example. Measures 3 5/8” x 2 1/4"
1920s Japanese postcard from a set showing scenes from the
Daredevil Jack serial.

Circa 1937 advertising piece for cosmetics products using Joe Louis photos as a promotional aid.
This Slim Jim Bows mail reply PC shows Marciano pre-championship, dating the promotion to 1951 or earlier.