Hers a few pictures to go along with the explanation
Below is my R. There are 4 bolts that hold the fender to the bracket
This is my X, There are also 4 bolts that hold the fender to the bracket.
If you notice the bolt pattern, one side longer than the other. for the R the long side is towards the radiator facing backwards. On the X this long side is opposite facing forward.
The fenders are interchangeable by rotating each 180 degrees then aligning the bolt holes on each of the fender brackets.
what is not interchangeable are the brackets.
The R bracket is much longer and faces the front of the bike to shield road debris from the inverted forks.
The X bracket is on the inside of the forks, and much shorter.
If your looking at a custom fender check the way it mounts, some to custom ones out there integrate the bracket into the design so a integrated fender/bracket for anything other than an R will not work, but will work for most every other V-rod 07 and up.