I found a thin 1961-D and compared it with another 1961-D and found it was smaller in diameter (18.6, while a normal is 19) and also smaller in width (1.2, while the norm is 1.6). Would it be a thin planchet (have you heard of such a thing?) or an acid bath?
Definitely very interesting looking, I don't know if it would strike up even that much if it was a thin planchet to start with, but I'm new at this.