John,
Your post sent me on a chase looking for a piece of Whipcord, about a 2" by 2" square, that I know I used to have. I must have loaned it out and never got it back, as it was not in the drawer it belonged in. But, here are a couple photos of a small piece of Whipcord from a Roadster top, around a snap. It is a little bit faded. I couldn't find a way to show the inner piece that is less faded. I know a couple people who frequent the board have larger pieces of Whipcord, but they're probably enjoying the end of the Hershey meet about now, or just about to start the journey homeward.
There's a photo of a piece of Whipcord on page 79 of "The Model A Ford as Henry Built It," by DeAngelis, Francis, and Henry. It's a black and white photo.
For your August '28 Roadster, I would use the black fine long-short grain material. There is some overlap and you could likley use Whipcord on that late of a Roadster, but that's pushing the timeline. Additionally, as indicated in other posts, finding Whipcord now is nearly impossible. If you're dead set on having a Whipcord top, you could contact Bill Sturm in Neenah, Wisconsin. I believe he has a little bit left, but it is NOT for sale outright. He may do your top, but he has also spoken of not adding any more cars to the list he already has lined up, as retirement is being considered, so he may not be an option at this point, I do not know for sure. Just trying to give you a lead if you were intent on having a Whipcord top.
Again, I would advise going with the black material on an August car.
Just my two cents, which we all know is worth about a tenth of that!
Dave Lopes