Rhode Island Revenue and  Postage Due Issue
Certificate Stamps - Revenue Issue of 1862 - 1871
Scott #R24 - Pair of 5 cent Washington Red
With New York, Providence and Boston Railroad (RPO) Cancel

This pair is a real find as it includes a Railroad Post Office (RPO) cancellation on a revenue (Certificate) stamp issue; a hard to find combination.  The cancel is an RPO from the New York, Providence and Boston Railroad and the stamps are Certificate Stamps, Scott  Nr. R24 from the first United States Revenue issues produced from 1862 to1871. 
See: Railroad Post Offices (RPOs)