Ciao Andrea,
ecco il responso dal pregevole e documentato forum USA Philamercury.
Gli annulli sono stati apposti a New York.
Riporto parte delle due risposte ricevute.
"Both of your covers bear New York Foreign Mails (NYFM) cancels. These were cancels applied at the New York exchange office to stamped covers to most foreign destinations that were sent from New York City. Your 10c Brown cover is the style commonly associated with NYFM cancels. The 15c stamp, although it also bears a NYFM cancel, is obviously a more plain style cancel. There is a premium value applied to the fancy styles used in the 1870's and really no premium for the style that appears on your 15c cover."
e
"These are NY Foreign mail markings, there are two major works on them.
1) The Foreign Mail Cancellations of New York City, 1870-1878 by William R. Weiss Jr.A comprehensive study that list over 200 markings and 2,000 individual covers,many stamps and covers are illustrated full size with extremely fine qualityhalftone cuts. Well printed and on heavy enameled paper.1991, 509 pages, cloth, edition of 500
2) New York Foreign Mail Cancellations 1870-1876 by van Vlissingen and Waud
1968, 105 pages, cloth, regular edition
1968, 105 pages + 11 transparent plates, cloth"
Luca