It looks like a Cataract 57, which means 5C collet and 7" swing. The serial number is on the flat surface of the headstock below the spindle nose. It will probably be something like 5-7-NNNN. There will probably be a different number on the right hand end of the bed. Sometimes there is also a number on the top of the slide rest, in front of the t-slot. You might have to get through some paint and rust to find the markings.

The 57 and 59 Cataracts are more valuable than the 37, 47, 49 or 67 models because the 5C collets and chucks are much more common than the smaller or larger sizes.

Tony's site - Cataract

Larry