Simply put the 48 frame is smaller in motor housing diameter than the 56 frame. If you look at the 4 bolts that run from the back of your motor and go all the way to the front and hold the plastic wet on -- if you measure across from side to side -the 48 frames will measure approx 3-9/16s, were the 56 frame motors will measure 4-1/8 inch across. So if your bolt pattern is under 4 inches -you have a 48 frame motor, if your bolt pattern measures over 4 inches you have a 56 frame motor.
This is important to know -specially if your ordering a wet end . The mounting foot is also going to be slightly different and sometimes the overall length can be different.
DOUBLE CHECK YOUR FRAME SIZE --USE A TAPE MEASURE as some pumps are labeled 56Y buy still measure the same as a 48 frame.