I just found this on CONECA's website:
Indented strike
When two planchets overlap each other in the coining chamber, the result is one coin which is a uniface off-center (sometimes called a “stretch strike” because the extra pressure forces the planchet into a larger-than-normal shape) and a second coin with a shallow, irregularly rounded depression on one side caused by the other planchet receiving the strike from that area of the die.
and I think your coin is what they referred to as the "second coin" in this explanation.
