The reasons I am not so sure are as follows: 1. If it was used as a button I would expect to see more wear to the edges and reverse. 2. It seems unusual to have the pontiff's head as the back of the button. 3. The material seems to be consistent with that of the coin. 4. The coin has spent a good deal of time in circulation with the abnormality present - the close up photo shows wear to the top half of the blob consistent with that of the coin. Used as a button then returned to circulation? Seems unlikely to me. 5. The die shows a couple of decent size breaks so the possibility of die degradation could be considered. 6. If this was a die defect then I would expect a concomitant weak area in the reverse, which is present. But it still looks ex-mount.