1890 2 1/2P / 20Pf Heligoland on Cover to Mainz, German States, Germany, Arrival Postmark, Last Month of Validity, Signed

1890 Cover to Mainz with 2 1/2P / 20Pf and typically postmark Heligoland from July 14th 1890 to Mainz with arrival postmark July 20th 1890.

The dates fall within the critical transition period of the Heligoland-Zanzibar Treaty, between GB and German Empire wich was signed just two week earlier on July 1st 1890. On July 14th Heligoland was still unter British administration, but everyone on the island knew the handover was imminent. The official handover was on August 10th 1890.

This cover represents the final month of the Heligoland's postal independance. It´s considered a "late use" of British-Heligoland stamps before the were replaced by standart German Empire stamps.