NEMA (National Electrical Manufacturers Association) frame size refers to mounting only and has no direct bearing on the motor body diameter. In any standard framenumber designation there are either two or three numbers.
56Y frame Square Flange motors are 1/2" wider (body diameter) than 48Y framebut have Identical hole spacings on the Square Flange. They are interchangeable. The Square Flange or Face, has 4 - Non Threaded bolt holes at 1, 5, 7 and 11 O'Clock to mate up with the seal plate. Motor Shaft is threaded on the end.