Pentwater & Nunica
Michigan
An illustration of your item would be appreciated.
From The Classic Society's "Chronicle", issue 67 of August 1970, p131, showing a tracing labeled as "partial" (and lacking the "AGT" at the bottom):

From Towle's 1986 volume "U.S. Route and Station Agent Postmarks", page 164:

Both list this from the same era, as used on Banknote or 1882.
Some caution should be taken in any tracing as missing or weak elements were often "completed" by the tracer, but that is a tangent for another day.