The spa and pool industry use various "Frame" sizes to mount their "wet end" or pump to. A bracket mount motor has a welded base for mounting. These motors can be a 48 or 56 Frame. Other types of motors such as the "C" Frame or Round Face and the Square Flange are all 56 Frame.There are several ways to figure out the Frame size. One is to look at the spec sheet where it says Frame (FR) it might have some letters with it but will have 48 or 56 as the frame size. The other way to determine is to measure the thru bolts (the 4 bolts that run thru the length of the motor and screw onto the wet end or volute of the pump). Measure the distance between the thru bolts, DO NOT measure diagonally but from one bolt to the next bolt.
48 FRAME distance between the 2 bolts is 3 5/8 inch
56 FRAME distance between the 2 bolts is 4 1/8 inch
Because all manufactures do not call all things the same this is the best way to tell the Frame size.

PLEASE NOTE: AQUA-FLO FMXP FRAME Wet ends, have two different sizes. They used 2 different motor manufactures, Emerson and Magnetek which are different sizes. Sundance used the Emerson from 1997-1999 on the Aqua-Flo pump they used:
EMERSON MOTORS: Distance between the 2 bolts is 4 inch
MAGNETEK MOTORS: Distance between the 2 bolts is 4 1/8 inch