Nicaragua 1903 Postage Due Unpaid Cover from Philadelphia to Bluefields, tied with Scott 151a

Unpaid cover without US stamps to which regular mail stamps were subsequently applied to meet the postage due rate on arrival. Of particular note is the 1901 20c on 5p black inverted surcharge in red (Scott 151a), cancelled with violet Bluefields oval, repeated on reverse together with New Orleans transit stamp. A scarce useage on cover, particularly with the inverted variety. Glue staining as per image.