FYI,
I own a GH-16ga that letters with 30" barrels and they measure 30" dead-on. The rib matting goes all the way to the end of the rib and the keels are in place.

This bit in the Parker story about the blank space at the end of the rib matting is not a hard and fast rule. I wonder how many nice 26" barreled Parkers have been passed over because the matting went to the end of the rib?