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davidcyclebackUPI was formed in 1958.
United Press (UP) or United Press Association were the earlier names.
UPI sometimes restamped old photos at later dates, but if there only is a UPI stamp that usually indicates it was made by UPI.
Even if with the 'printed later' images, each UPI photo is still limited in number and collectable. UPI owned and had exclusive rights to the negatives, so the images are often nice sharp and clear.
The rule of thumb identifying vintage photos from news services is to look for the stamps of UP, United Press Association, ACME Newspictures, International News Photos (or similar International News names), Bain News Service or George Baine, Pacific & Atlantic, Keystone View (not a complete list). These news services went out of business or changed their name years back and their name can only appear on photos from the 1950s or earlier.
Brown paper caption tags generally are vintage, as the tags were originally brown and took many years to fully change color.
For more more modern wirephotos, if the caption strip is in the image (actually part of the photographic image as opposed to being phyically taped or glued on-- you can see it but can't feel it), the date of the caption is reliably the date the photo was made; ('Associated Press: June 10 1953, NYC: Mickey Mantle steps to the plate...').
If a date is stamped on the back of the photo, that is almost always the date it was stamped. Vintage date stamps are not common but desirable as they help prove age.
I own a collection of Vogue magazine fashion photos. Say what you want about the magazine, but their original 1930s-40 studio photos are usually stamped with photographer's name and the day/month/year the photograph was made. This makes life easy for the person trying to identify and date the photos ... What I find interesting about Vogue magazines from the period is that they credit the photographer and sketch artist for each photo, even in the advertisments. This is most unusual for magazines of the period and must be a comment on their feelings towards the photographer. Almost no 1930s-40s ACME, ACME or UP caption tags list the photographer.