@Cyber-Robin:
Looks like Margaret/Boa Hancock's base from the last series.
Kind of what I was thinking, though I checked Margaret and Hancock's bases and they don't quite line up with this one (Hancock's came closest though.) Nor do Star Hero Ace or Luffy's. The hole where the cape support goes is hexagonal, if that helps narrow it down for anyone who might know.
@sensui:
Glad to hear things got to you ok, and I think you're putting it on wrong, look at my photo @ Mihawk for the proper orientation:
! http://i980.photobucket.com/albums/ae285/sensui123/Misc/One_Piece%20Stuff/42e4a92d.jpg
Thanks, I tried that but my base is too different to achieve that orientation. Whereas your Mihawk's cape support is at the edge of the base, mine's toward the center. That's a big reason why I think that this might be the wrong base altogether.
EDIT: okay, mystery solved. It's a Zoro base, as I just opened my Zoro from this set and their bases are exactly the same, except that Mihawk still came with his correct support (Zoro's is a long peg that straps to his thigh.) Here's a comparison pic:
!
! Zoro on left, Mihawk on right.