Thanks guys. I tried once more, using my large woodworkers vise and heavy gloves, and got most of the curve out. I think I'll be able to get the rest out the same way, but if not, I'll go to the heat treaments you have suggested.
It came that way, but everything else in the package was fine. Pretty much corrected now. Any residual fits in nicely with the folk artist persona I'm adopting. If my work is bad enough, it'll probably be worth a fortune!
I have flattened them out by setting them on a anvil and putting a piece of drill rod stock in the groove to pound on and move the rod down the groove as I persuade it with a hammer. Do it cold. The devil won't get ya. It seems every rib I ever bought has a little curve down to it and tends to hump up when ya lay it on the barrel flat.
Don