David,
Your cancel is a Flaggenstempel (flag cancel) of the type used from 1 July 1928-1945 on ferries of the Gjedser-Warnemünde Line, which ran a ferry service between Gjedser (modern Gedser), Denmark, and Warnemünde, Germany.
There were four major versions of the G-W Flaggenstempel cancel. The initial three cancels depict the ferry from the starboard bow.

These three can be distinguished by the letters preceding the date:
"F.b" -- used on the first trip of the day
"F.h" -- used on the second trip of the day
"F.g" -- used on special, unscheduled trips (rarest variant)
Yours is the fourth variant, introduced in 1930 and used until at least 1943. It primarily differs from the previous versions in that it depicts the ferry from the stern. It's only known with the "F.h" letter combination.

Catalog for a legitimately used, fully intact cancel of the fourth type on postcard is about €70.
The ship in the Flaggenstempel is the railroad ferry,
Schwerin.


The
Schwerin is also depicted on a German stamp, Michel 657 of the
1937 Winterhilfswerk Issues.
