My father-in-law was cleaning out his basement recently and discovered a cache of slide carousels taken in the 1960s that his deceased wife left behind. He sent them to my wife and I and we have had them scanned and are cleaning them up now. The question I have is what the intended aspect ratio was? Some of the slides are clearly 3:2 and have no border around them. However, a large batch of them appear to be almost, but not quite 5:4 with a black border to pad them to 3:2. They are just slightly too tall to be 5:4. Was this just some "slop" that the format supported or was there some other well-known aspect ratio that they represent? (They also aren't 1:1.)
There's also some black fringing on the sides that makes it look like torn paper. The original images are about 3120x2048 pixels. Once I've cropped them to 5:4, they are around 2472x1976 pixels. (It's inexact because I'm cropping each one separately by hand.) So there about 72 pixels of extra height, or about 3.5%. What's the reason for the extra? Or is it not extra, and I'm missing something?