In the 50s it's likely to have been a water-based glue. If you want to take the risk, place a photo, black paper down, onto water. (You could use something like a baking tray, or just a plate, if the photo's small). Let it float, and try not to get any water on the front of the photo.
Leave it to soak, maybe for an hour or more, and test until the photo separates.from the backing. Once removed, mop the photo with a kitchen towel and set it aside to dry.
If the photo curls up, you can iron it flat.
One problem is that if anything has been written on the back with fountain pen ink, you'll likely lose it.
The water shouldnt harm the photo paper - it's made to be developed in liquids and be washed afterwards. But don't soak it for days!
If a balsa cement was used - you have a problem!