I have two color photos from Crete, Greece. Maybe around the 1960-70's? The size of the photo is about 3-1/2" x 5". I believe it had a white border but I think someone trimmed it. There is only a long number that is printed almost the full width (about 2-3/8" across): 034020540 and the other photo 0341001(0, 6 or 8)2. With spaces between each number. How can the photo be dated closely? Thank you.
These numbers were probably added by the company that printed the photographs. Their purpose is usually to record which job the prints belong to, where a "job" is a single film's development and printing. With hundreds or thousands of films being processed each day in an automated plant, tracking numbers were very helpful if there was any mix-up. The prints were put into paper wallets by hand at the end of processing, and mix-ups were possible there, as well as if there were problems in the automated parts of the process.
The numbering scheme would be specific to the company that did the work. Using it for dating would require identifying the company, it still being in existence and having all its old records, and being willing to go to the trouble of looking them up. Since many photo processing companies have gone out of business with the rise of digital photography, the odds of success are not good.
Source: observations from working in a camera shop in the 1970s.