FDC servicers William Siebold and Henry Hammelman knew each other in their hometown of Buffalo, New York. Both ended up working in Washington, DC; Siebold with the U.S. Patent Office and Hammelman with the Post Office Department. Some of Hammelman’s earliest FDCs (1913) were addressed to Siebold and Siebold’s daughter, Marie. Siebold would service his first FDCs in 1920 for the Pilgrim Tercentenary Issue (Scott 548-550). Some dealers confuse Siebold and Hammelman FDCs since both frequently addressed covers to the two Siebolds at their home address. I have a January, 1950 FDC made by Siebold for the Samuel Gompers issue (Scott 988).
Even though this cover has a return address to Clara Helff, a Siebold associate at the Patent Office, it is definitely addressed in Siebold’s handwriting.