This is one of the very few (possibly unique) strips of 3 of the 5c 1856 imperforate issue cancelled with the iconic large Boston PAID in Grid cancels.
The 15c rate is for 0-1/2 oz. letter sent on Dec. 22, 1857 from Boston to La Rochelle, France, per the U.S. postal agreement with France of 1857. The GB 1F 60c marking indicated the bulk rate per 30g. Ex-Hackmey.