-countermark-

n. 1. A part corresponding to another part; anything which answers, or corresponds, to another; a copy; a duplicate; a facsimile. [ 1913 Webster ]

In same things the laws of Normandy agreed with the laws of England, so that they seem to be, as it were, copies or counterparts one of another. Sir M. Hale. [ 1913 Webster ]

2. (Law) One of two corresponding copies of an instrument; a duplicate. [ 1913 Webster ]

3. A person who closely resembles another. [ 1913 Webster ]

4. A thing may be applied to another thing so as to fit perfectly, as a seal to its impression; hence, a thing which is adapted to another thing, or which supplements it; that which serves to complete or complement anything; hence, a person or thing having qualities lacking in another; an opposite. [ 1913 Webster ]

O counterpart
Of our soft sex, well are you made our lords. Dryden. [ 1913 Webster ]