i can confrim this synth. i just made 2 windurst doublets and a single fed doublet. the material list is just as ichimaru said it is.
my cloth lvl is 53 when i did this, considering it is cotton, i think you should start to make this at around lvl25 or so, just as the cotton skill maxes out. considering that i got an HQ sythn from it, it makes sense.
Windurstian Doublet is the upgrade from Mercenary Captain Doublet, and the HQ from that synth is the +1 of Windurstian, the "Federation Doublet" the recipe for Windurstian is on the website, i believe
enjoy!
Edited: Oops! there's no recipe... then here it is: Clothcraft(??) Earth Crystal Cotton Thread Cotton Cloth Mercenary Captian Doublet
i've never synth this as of yet, but that's what i gathered from JP website