Continuing with the seaplane posting streak, a rather rare photo of a Kawanishi K-7A, registered J-COBC. As wiki says (HERE) "The K-7A entered service on Nippon Koku's service between Osaka and Fukuoka in January 1925, mainly carrying airmail rather than passengers. The K-7A proved successful in service, one carrying mail from Fukuoka and Shanghai, China, a distance of over 950 kilometers (590 mi) in May 1926...".
Note the mail marking behind the civilian registration. As wiki again explains: "The symbol of a post office in Japan is a stylized katakana syllable te (テ), 〒. This is used on the signs of post offices, on post boxes, and it is also sometimes used before the postcode on letters. This mark is derived from the Japanese word teishin (communications)."