What you have is a re punched 8's. Almost all 1888 have at least 1 or 3 of the 8"s re punched. Another variety of the 1888 is called a bead punch where the middle 8 has a half moon on the lower loop on the left side. The working die has been repaired by the engraver and because of this repair the middle 8 shows this repair. The 2011 Charlton Catalogue in the back of the book has some of the variety's. I have not heard of any published book just dedicated on 1888 variety's to date. My advice to you take notes keep on looking because you never know what you might find.