The GR series has the rib extension. This is a 424 receiver, no extension.

The other differences, bluing / plating, etc are merely cosmetic. As to the product name (type of steel) on the barrels, they could and did change that from year to year. Again, they offered a sightly different product in each market.
Wilson is a good survey. He should not be taken as the last word with regard to Beretta. He had a huge task, attempting to document over 400 years of weapons. In addition to giving a rough sketch of the history. I think it is better than some of his other books.
Most of the guns Wilson shows can only be seen in a museum or private collection? When was the last time any one saw a Beretta shotgun from the 1880's up for sale? I have "heard" of them changing hands, but only through private channels. Heck most of the catalogs he shows will never see the light of day!
Pete