DF-500, DF-550
The difference between two models is only length of Buck body.
DF-550 is longer than DF-500 with 120mm, it may be an improper expression DF-550 is for European, American. And DF-500 is for Asian people.

DF-730, DF-740
The difference between two models is only height of machine which effect.
Pants length to be applied, DF-740 is also available even for longer parts than DF-730.