Watch List notes:

UNCERTAIN. Æ (30mm, 22.69 g). Countermark applied mid 1st century AD(?), or in modern times. M shaped countermark applied three times (or MM?) on a sestertius of Marcus Aurelius as Caesar. Apparently unpublished in the standard references, but cf. CNG E-434, lot 196. Host Fine, c/m's VF.

From the Richard Baker Collection.

This countermark is quite possibly modern in origin, considering its appearance on sestertii issued over a century apart.