Hey Jaster-
I'm with you on this... I can only see the differences in the canopy and the addition of another radio mast. According to the book I'm looking at the tail-hook leg (the little nub that extends behind the vertical and horizontal stabo's where the tail hook comes out) is a little longer on the 1A and 1D compaired to the 1.
There were some engine differences too, with the F4U-1 having a P&W R-2800-8 (2000 hp) and the F4U-1A &1D using the P&W R-2800-8W, a water cooled engine that put out 250 more hp.
The visual differences between the 1A and 1D seem to be minimal, with the exception of the 1D having additional hardpoints for bombs and whatnot. As far as I can tell the 1A and 1D used the same canopy...
Since I build in smaller scales I can get away with more, but if you're building a big-bird you may want to pick up a copy of squadron publications corsair in action
http://www.squadron.com/ItemDetails.asp?item=SS1145The book I got my info from is called the "Encyclopedia of aircraft of World War II" It's a bargain book that you can pick up for about $20 at Waldon books or Barnes and Noble. ISBN # 1-904687-07-5 it's a good book with a little information on a lot of airplanes (Axis and Allies). If anything it's fun to thumb through while watching the History Channel. :-) :-) :-)
Good luck and keep us posted !
-Scott