Nice, I have 40 Mercury dimes myself, I also have 100 walking liberty half dollars and an ancient Roman Denarius and 1 Ounce Australian 2013 year of the snake and a MS67 Quarter and 1 Silver certificate. Thats my whole collection so far.
It's one dime from a set that I purchased that was in a cardboard display. The dimes in this display were all toned similarly. Probably toned due to the environment the original owner had them in. Maybe he/she kept them in a basement or garage? Don't know.