Thanks for the response, Joe. The panels are tan. No markings, just a little barcode sticker that says they were made in Canada. I saw what appears to be the same thing at Lowes. No ID there, either, other than "soundboard."
The garage is already sheetrocked. I'll just be installing the soundboard over that. I did find installation instructions for a product called Homasote, which stated that there should be a 1/8" gap on the long dimension, and a 1/16" gap on the short one. I think I'll go with that.