The reference in The Chronicle includes an excellent article by Susan McDonald suggesting the reason for the presence of the U.S. stamps. This is a stunning cover, worthy of gracing any important collection
cover 6p olive-yellow (2; S.G.4), two singles used with United States 1857 1c type V and 3c type 1, tied by red "PAID" h.s. to May 1859 cover from St. John to Liverpool, showing red Colonial Express Mail St. John N.B. May 23 d.s., red paid Liverpool Br. Packet 4 JU 59 d.s. and red "19" h.s.; the stamps with some faults, the cover a lovely Express item
Robson Lowe: The Encyclopaedia of British Empire Postage Stamps, Vol 5, illustrated p. 326, 1973.
The Chronicle, No. 134, Vol. 39 No. 2, p. 145, May 1987.