I think more recently Dave started placing his sticker logo on the bottom, or signing them, but before I have had Dave mod a module, and a friend's as well and it wasn't signed or anything. I sold one, and the buyer had been taken before, so he requested sending a gut shot picture to Dave to verify his work, which he was able to do.
So I wouldn't be too concerned with not having any signature. Sometimes the mods, unless done to completely change the module, are more subtle improvements over stock, something you might not pick up unless you A/B'd with a stock module, but often times the little improvements are what its all about.