Postcard with bisect 4 cent - pen canc. (tied), purser S/S CANADIA and ms date 23/III 1903, to Germany.

Covers mailed while on board a German ship 

A ship´s purser could keep a supply of stamps from the country where the ship departed, and under the UPU rules he would post them in the first port of call where they would be accepted without penalty. 

This practice is allowed today, a century later. The German steamers han their own cancels with the ship´s name that somtimes would be used to cancel the stamp, but other times the purser would use a pen and ink to write the date on the stamp. Other times the stamp would just be marked with an "X" with the date written on the envelope or postcard. 

All German/DWI steamship covers are highly sought, and I am aware of five postcards having a bisected 4-cent stamp and used on postcards mailed to Germany.

Frontside of the postcard - Note - dated (Le) Havre on April 10 from which the postcard is sent to Germany

Bisected and single 4 cents used on a fragment from a cover. Pen canc.
Purser “Polaria” - date May 23 -1903. The final at St.Thomas. Dot after the T - flaw.
The German ship “Polaria” was in St.Thomas in May 1903 (St.Thomas Tidende). The ship sailed to Mexico - Tampino. Covers carried via the Hamburg-Mexico line that called at St.Thomas during 1901-03 could exist, bud none are recorded.
It was stamped on a German ship, and it seems that they accepted the bisected stamp as two cents. They did not know that a bisected stamp could not be used with other stamps, and they could not bring a postage penalty.
It was acceptered as 2 cents. It's probably the only time that we will see it. It is unique.
A ship can not send a letter Recommended / Registered. It can only be undertaken at a post office.
A foreign letter cost 8 cents, the fragment is missing one or more stamps, and you can only speculate and dream about what they were.

St.Thomas, February 27 to Friesac in Germany. Delivered aboard the Hapag steamer “Sibina” marked by manuscript. Arrival March, 21.
(Frank Banke)