What size is your pin? Hake's had another Fitzsimmons pin in 2008 that they described as:
1.25" listed in "Boxing Pinback Buttons By Muchinsky" as #12. Muchinsky notes this and the following 2 buttons were likely issued in anticipation of the famous 1897 Corbett-Fitzsimmons fight. Lower half of the backpaper lists the W&H name and 3 early patent dates but the upper half of the paper has this text "Wm. F. Miller & Co.,/23 Duane St./N.Y. City."
http://www.hakes.com/item.asp?Auction=196&ItemNo=77145
No back image, but the verbal description appears to match yours. Perhaps yours is the other of the 2 buttons described in the book? I do not have Paul's boxing pin book (his baseball pin book is amazing though, so I probably should), but perhaps someone else does and can confirm or deny...?