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On May 27, 1908 Congress approved and provided guidance to have postal clerks on board U.S. Navy Ships. On June 27, 1908, the Acting Secretary of the Navy, J. E. Pillsbury issued General Order No. 74. This allow for ships having a complement greater than 650 men to have one Navy Mail Clerk and one Assistant Navy Mail Clerk. For ships with a complement greater than 125 men it authorized one Navy Mail Clerk. They were required to take an oath required by section 391 of the Revised Statutes for the U.S. Postal Service. The first post office being authorized for the USS Illinois effective the day after she left San Francisco on her voyage westward.
Though authorized in 1908, it was not until the 1920s that a U.S. Navy system was established to classify these postmarks. The "Locy" system is the basis for postmark classifications in use for all Naval Cover collectors. The Type I postmark was available to naval units in 1909 and by 1911 the Type II postmark.

The cards of my collection are presented in no particular order. They are hard to find and I generally try to pick-up one when I see it. I use the Catalog of United States Naval Postmarks, fifth edition to determine the relative scarcity of a particular postmark. The cards I have posted to my site are all from the last portion of the cruise, when they received their postal stamps in early January, to when they arrived back in Hampton Roads, before February 22, 1909. I show all of these because i believe they are the most scarce postmarks to find. In some cases, as with the Rhode Island, I show the earliest cancellation that I have found, even if it is after their arrival in Hampton Roads. It is not clear to me that all of the ships had certified post offices or had received their cancels prior to arriving back in Hampton Roads.
Earliest gwf postmark
Correct me if you know of an earlier one. The USS Maine, which was dispatch from San Francisco with Alabama ahead of the fleet. Postmarked December 12, 1908 on a nice Japanese card. The card is sent by by Alfred to his sister. From the card it appears they were in New York getting ready to go south to Cuba.
An interesting card!
USS Illinois
Postmark: January 21, 1909
Postmark on a Cairo, Egypt card showing a merchants.
"Many thanks for pretty card just received. Sincerely, A. C. Neweman, USS Illinois."
Listed as SCARCE
USS Georgia
Postmark: January 21, 1909
Postmark on Marseilles, France card showing harbor.
"Hello Ed, Had a very stormy run through the Mediterranean. Arrived here 1-15.09. Expect Fridays leave here may go to Paris. Home in March. Maybe come to Easton. A. Thompson
Listed as SCARCE
USS Kentucky
Postmark: January 21, 1909
Postmark on Alger card showing the place of government.
"Happy you are enjoying the best of health, Yours Will, Algers 1/22/09"
Listed as R-1 (from 25 to 100 copies recorded)
USS Minnesota
Postmark: January 14, 1909
Postmark on card from Cairo, Egypt showing the pyramids.
Mailed to Chicago, Ill
Listed as SCARCE
USS Wisconsin
Postmark: January 4, 1909
To my knowledge, this would be the first day of the postmarks use.
Mailed on a card of Port Said, Egypt showing the waterfront.
"We're now in Egypt. Egypt is on one side of Suez Canal and Arabia on the other. We had a time in India. We did not have a very big time on Christmas. From H. B. Mosher'
Listed as SCARCE
USS Virginia
Postmarked January 5, 1909
To my knowledge, this would be the first day this postmark was used.
Mailed on a card from Cairo, Egypt - Confectioner's Shop
Mailed to Green County, New York
Listed as SCARCE
USS Virginia
Postmarked January 5, 1909
To my knowledge, this would be the first day this postmark was used.
Mailed on a card from Cairo, Egypt from the same sailor as above - Ahmeda Palace in the Fish market
Mailed to Green County, New York
Listed as SCARCE
USS Minnesota
Postmark: January 14, 1909
(same sender as above card)
Postmark on card from Suez Canal showing German merchant transiting the canal.
Mailed to Chicago, Ill
Listed as SCARCE
USS Kansas
Postmark January 1?, 1909
Card from the pyramids and sent to Cleveland,Ohio
"A close-up view of the pyramids, Sam"
Listed as SCARCE
USS Kansas
Postmark January 12, 1909
Nice card showing the merchant activity on the Suez.
Listed as SCARCE
USS Kansas
Postmark January 5, 1909
To my knowledge, this would be the first day this postmark was used.
This sailor on board the Kansas was still catching up on mail and sent an card from Ceylon showing an Elephant Kraal
Listed as SCARCE
USS Georgia
Postmark January 15, 1909
A Tucks Postcard of a "street scene Cairo"
"January 12, 09, Cairo is a city not far from Port Said that is where these coins I sent come from. Yours, W. W."
Listed as SCARCE
USS Wisconsin
Postmark January 9, 1909 (see front)
Postcard of Port Said - Maison Hollondaise
Listed as SCARCE
USS Virginia
Postmark January 5, 1909
To the best of my knowledge, the is the first day of the Postmark usage.
Postcard from Egypt showing a traditional cafe and donkey rider.
Listed as SCARCE
USS Missouri
Postmark February 23, 1909
This one I let slip in only because it had the rock of Gibraltar.
"Received your letter at sea. This am. I trust these cards mine be of interest to you. C"
Listed as SCARCE
USS Georgia
Postmarked February 1, 1909
This is a rare card from Tanger with a ship postmark.
"January 31st. Arrived yesterday morning and without regrets leave today for the rock with the Prudential Insurance Company sign. JLH"
Listed as SCARCE
USS Wisconsin
Postmarked January 30, 1909
Postcard from Alger, "Algeria Jan 29th, I received your letter some time ago. I am having a good time at present. H. S."
Listed as SCARCE
USS Connecticut
Postmarked February 3, 1909
Postcard from Gibraltar from the Bay
"How is everything in Chicago. Cousin Watter, USS Connecticut
Listed as SCARCE
USS Kentucky
Postmarked January 18, 1909 (same sender)
Postcard from Tripoli - very rare
Listed as SCARCE
USS Missouri
Receiving Postmark January 8, 1909
Probably the only one in existence. Uber rare.
Thanks Sophia for the card of the old National Bank in Centralia!
USS Kansas
Postmarked January 12, 1909
Postcard of the rail station in Suez, Egypt
"From your Uncle Alex, USS Kansas"
Nice - Clean - Card
Listed as SCARCE
USS Nebraska
Postmarked January 16, 1909
Postcard from Port Said, Egypt, Entry to the Canal
A card to Susie discussing shipping in the Canal.
Listed as SCARCE
USS Kansas
Postmarked January 25, 1909
Postcard from Nice, France of the Promenade de Anglais
Mailed to Mr. F. E Biederman, from Bill
Listed as SCARCE
uss Rhode Island
These two cards, one from Egypt, the other from Gibraltar, were both canceled back in Hampton Roads on February 25th, 1909. However they are the earliest postmarks that I have found from the Rhode Island and I wonder if the postal clerk did not get certified until returning or if the actual rubber cancel had not been received until returning.
Both cards are addressed to the same "George Pritcher of Norwich, Connecticut" and contain messages that suggest he was back home at the time the cards were sent.