REGISTERED / CAVITE, P.I.
Unlisted Purple Registry Postmark

1902 JULY 11: CAVITE to DETRIOT
From the U.S.S. Isla de Cuba, franked with a pair and 3 singles of a US 2c carmine Washington booklet pane (214b) for 10c Single Outbound Registered Rate (2c + 8c Registry Fee), tied with segmented cork killers, CAVITE purple registry dispatch on the sde. on the sde. Reverse with  purple d/r RECEIVED / JUL 12 / MANILA P.I. cds, and Detroit, August 19th, 1902 d/r cds cancels.
 The 3-line REGISTERED / JUL 11 1902 / CAVITE, P.I. registry marking is not listed by Goodale and Baker, the latter lists a 4-line marking with "Military Sta. No 2, Cavite / Philippine Islands" as the 3rd and 4th lines of the postmark shown below.
Originally the Spanish Isla de Luzon class cruiser, the Isla de Cuba was launched at Newcastle on the Tyne in December, 1886. Part of the Spanish squadron in the Philippines the ship was sunk in the Battle of Manila Bay on May 1, 1898. 

Salvaged, the ship was commissioned as the USS Isla de Cuba in 1900. Sold to Venezuela in 1912 the ship served as the Mariscal Sucre until scrapped in 1940.

1902 DECEMBER 8: CAVITE to BOSTON
The same postmark with CAVITE, P.I. in a higher position relevant to the date
and shifted to the right if compared to the above cover.
From the two examples there is reason to believe that a separate device
was used for the "CAVITE, P.I." line.