The first thing to jump out at me is that the noses are different. After a closer look, I must ask, is the one on the left a doubled die?
Well, I don't collect them, but the earflap of the cap looks slightly different as does the hair detail along the cap. The tip of the nose appears to be turned up slightly on the left-hand specimen. Chris
I don't pretend to know about the series in this detail. My only comment would be the one on the right looks like it has been touched up, (details strengthened, happened a lot with new issues). So, either the one on the left is earlier, and the mint touched up the hub, or if the two are from the same hub the one on the right is from a fresher die.
The coin on the left is a 1916. You can tell because the rim is struck too high. It is called a "fin".
Flat, square rim on the dime on the left (flat spot on hair detail). Rounded rim on the dime on the right. Is the dime on the left a proof and the dime on the right MS?
That's the 1st thing that hit me were the square rims , but Mercs were brilliant proofs if I remember correctly , this ones fields look MS . The wing is larger on the right coin , since I have no books on Mercury dimes . I have no idea what to look for .
Supposedly Wallace Stevens' wife was the model. I read somewhere that there were several different strikes in late 1916.