Here is how you can date real photo postcards that have not been mailed/canceled

Look at the back of the card where the stamp would normally go, you will see a name or figures there, below match that with the dates they were used

AGFA ANSCO 1930'S-1940'S

ANSCO TWO STARS AT TOP & BOTTOM 1940-1960

ARGO 1905-1920

ARTURA 1910-1924

AZO SQUARE SQUARES IN CORNERS 1927-1940'S

AZO DIA DIAMONDS IN CORNERS 1907-1908

AZO TRI 1 4 TRIANGLES POINTED UP 1904-1918

AZO TRI 2 TRIANGLES TWO UP TWO DOWN 1918-1930

CYKO HOLLOW LETTERS 1904-1920'S

CYKO 2 SOLID LETTERS 1906-1908

DEFENDER 1 DIAMOND ABOVE AND BELOW 1910-1920

DEFENDER 2 DIAMON INSIDE 1920-1940

DOPS 1925-1942

EKC 1945-1950

EKKP 1904-1950

EKO 1942-1970

KODAK 1950-

KRUXO 1907-1920'S

NOKO 1907-1920'S

PMO 1907-1915

SAILBOAT 1905-1908

SOLIO 1903-1920'S

VELOX 1 SQUARES IN CORNERS 1901-1914

VELOX 2 DIAMONDS IN CORNERS 1907-1914

VITAVA 1925-1934