This first experimental postmark is known from Yokohama only. It was used for a brief period: from June 12 1878 to March 27 1879. It has a 27 mm outer and a 16 mm inner circle and was struck in magenta color. A similar looking cancellation from Hakodate was reported in the Philatelic Magazine in 1956. In case it exists it unquestionably is not part of my collection.
 
 

3y1: Cover with a 10 Sen old Koban blue, from Yokohama to Alresford, England  Foreign Mail Daub Killer Early Double-Circle Yokohama: Aug 16 1878 San Francisco: SEP 2 Alresford B SP 20 78 Remarks: I couldn't help bringing out the cancellation by enlarging and digitally enhancing the image.

 

3y2: Cover from Nagasaki to Brooklyn, N.Y., with a 5 Sen old Koban stamp.  Nagasaki Crossroads killer type NS79A Nagasaki Early Large Circle with month, day and time indicia: FEB 1 12M. Yokohama Early Double Circle with year slug at the top: 1879 15 FEB San Francisco Paid All marking: MAR 5 Receiver mark on the back: Brooklyn N.Y. MAR 12 8AM REC'D

 

3y3: Cover from Yokohama to New York with 3 off 5 Sen old Koban stamps. This must have been a heavy letter because the rate to the US was 5 Sen per 15g only. The strip of 3 stamps in paper type A have a 12 1/2 perforation.  I present this letter because the year in the Yokohama mark looks like 1878, but this cancellation was first used on June 12, 1878. The city of Peiking was sailing on March 5 1879 - so I am twice wrong with 1878. Yokohama Early Double Circle: MAR 5 1879 PAID ALL and New York receiver on the back.