After the formation of Yokosuka Kokutai the Navy decided to continue its aviation expansion program. At the tip of the Nagasaki peninsula the shallows were landfilled and an airfield was created. Sasebo Kokutai was formed on December 1st, 1920 using the airfield for its base but since it was too short, land-based aircraft of the unit were actually based in Oomura. As a result SaseboKu became a primarily seaplane unit.
On December 15, 1944 local anti-submarine patrol units were incorporated in three big kokutai. West Japan became the responsibility of the 951st Kokutai which included SaseboKu, MaizuruKu, OkinawaKu, 453Ku, 256Ku and ChinkaiKu (responsible for the patrol of Korean peninsula and located in today's Jinhae-gu in S. Korea). The other big unit was the 903rd Ku which was responsible for East Japan and the Tokyo area. The last big kokutai was the 901st which was responsible for the mainland and the areas of Asia occupied by the Japanese Navy.
The NARA photo below taken at the end of the War shows a Mitsubishi F1M "Reikan" (Pete) with katakana "サ" (SA) on the tail, the SaseboKu unit marking but also the number 951 underneath denoting the 951Ku.