Stamp box directory
EKO stamp box: dates and marks
If the stamp box on the back of your real photo postcard carries this mark, the paper's production window narrows the card's date. A stamp box dates the paper, not the photograph — read the range below as “no earlier than,” and let a postmark set the latest possible date.
Standard mark
1942-1970
Ranges cross-checked across: the2Buds.com chart, as republished by Sports Collectors Daily, 'Dating Vintage Real Photo Postcards'; SABR Baseball Cards Research Committee, 'Dating and Authenticating Real Photo Postcards' (2017); Playle's Directory of Stamp Boxes (playle.com, frozen reference; marketplace closed 2020). Approximate by nature — see methodology.