I've posted this pic a number of times, but it shows the differences in freehub body rear flanges and UG/HG vs HG.
The top row are the thick flanges. Left is a 7 speed UG/HG off an FH-M732, right is a 9 speed HG off an FH-7700.
Bottom row are the thin flanges. Left is a 7 speed UG/HG off an FH-MT62/M650/1055. Right is an 8 speed UG/HG off an FH-6402.
John