Charley wrote a quip for most of the serigraphs that he produced with Frame House Gallery and Mill Pond Press. Some were printed on small sheets of paper that could be framed as companion pieces. (Some did not), but all of these little stories can be read in his book, Beguiled by the Wild.
A cardboard back is not a good sign as to indicating that the other components are preservation quality. Is the bevel of the matting pure white of brownish yellow? The piece was issued in 1971 and framing pratices were not always great in the 1970's.
I would estimate that this serigraph might be valued at $300-400. Please feel free to Email me directly if you have any further questions.