I believe, although I could easily be wrong, that the Pluto chip is a pre-1911 chip. 1911 is the earliest year in which I hve a USPC chip in which the inlays were securely set (ie where the c&s was done right!)

I have (at least) 2 other chips with a Pluto-like inlay: a single flag (Union Jack one one side, stars and stripes on other) that shows a reg. date of 1896 I believe. The other chip is the jockey chip. I have 2 jockey chips depicting 4 jockeys. All 4 rode between 1881 and 1908. I believe that the reason for the lifting inlays and stains is that the inlay sandwich used to make the inlay was paper and celluloid. The tension from outer celluloid layer was not matched by the paper underneath resulting in the warping or lifting of the inlay. Being paper, the underneath stained when it got wet as can be seen in the Pluto chips.

After 1910 celluloid was used in both slices of the inlay sandwich. Tension was evened out resulting in no inlay uplift.

This is my theory until I hear better.