Registered mail exist only in "fronts".
The registration system required that once the letter is delivered, 
the front was removed  and the addressee had to sign on the back
that he had received the contents.
The front was then returned to the sender as proof of delivery.
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1817 JUNE 18: MEXICO to TIXTLA
FRANCO / EN MEXICO in two lines and fancy oval framed
R / INTEDENCIA / DE MEXICO markings.
Registered with circular corner markings and CERTIFICACION 
A MEXICO / SALIO EN 18 de Junio de 1817 (the date in manuscript).

PHILIPPINES to EUROPE "GALEON DE MANILA"

Spanish authorities approve the trade with the Philippines 
only through the Mexican port of Acapulco. 
Trade was carried out once a year by the Manila Galleon. 
After 1734 more ships were authorized  to carry the trade which 
continued until the beginning of the 19th century, when free trade 
was authorized and different ships started to use the
Cape of Good Hope route.

 1794 APRIL 5: MANILA to ROME

Via SAN BLAS on the Pacific Coast and VERA CRUZ 
on the Atlantic Coast. 
Black S. BLAS in oval alongside red NUEVA / ESPAÑA
on arrival in Vera Cruz. Charged "3" reales on arrival. 

ONLY KNOW COVER SENT VIA SAN BLAS