I have a color photo from Crete, Greece. Maybe around the 1960's? The size of the photo is 4-1/2" x 3-1/8". There is no border around the front. On the back, there are no printers logos. The only thing there is a long number that is printed almost the full width (about 2-3/8" across): 034020540. With a spaces between each number. And before the first number, 0, there is some kind of rectangle printing mark. I also have other photos that are black and white that have similar long numbers in the back. How can these be dated closely? Thank you.
The photofinishing industry begun automating film developing and printing starting well before 1940’s. Worldwide, these were not mini-labs, they were regional labs located in principal cities. By 1960 this industry utilized assembly line procedures.
Customers took their film to drugstores and camera shops for developing. Clerks accepted films and placed one roll in an induvial empty bag. Clerks wrote the customer’s name and filled out pre-printed instructions on each bag. instructions were, number of prints from each negative and desired print size etc. Each such location was assigned a location number and a delivery route number.
When the bag arrived at the lab, a numbered adhesive label was affixed to the bag. Each label had a duplicate with the same unique number (twin check system). The bag with its roll was sorted as to which developing machine room was appreciate. In total darkness, a machine operator unrolled the film and affixed the matching twin check to each. The film was then developed and emerged from the machine and matched to its bag via its unique number.
The bag and now developed roll of negatives was sent to a printing room. The film was manually threaded into a printing gate and a mechanical counter was dialed so that it displayed the unique twin check number. During the printing operation, a mechanical rubber stamp printing this number on the back of each print as it was exposed by the machine.
The now exposed print paper, in one long roll, perhaps 500 or 1000 feet long arrived at an assembly station. The prints were semiautomatically cut to size. The roll of film was also cut semiautomatically cut into strips, Prints, film and bag were thus matched by the assembly station operator. The bag with its now developed film and printer pictures was sent back to camera store or drugstore via a route driver.
Bottomed line number on the back of the print is the twin check number and not a date. In that era, if a date was printed, it was photographically applied in the margin of the print, exposed as the image was printed. This is a large number because a regional lab handled thousands of rolls per day.
You probably can't, unless you can contact the shop that originally developed the photos. And that is highly unlikely these days, as most shops that used to develop film or sent it out for development no longer do so, if those locations are even open.
The number on the back of the print is likely a serial number or counter of some sort, perhaps with some more information encoded in some of the digits. But unless you know the brand of developing equipment, or at least some information about who provided the service, you're probably out of luck.