From:
John Leach's Encyclopedia of Postal Authorities :

K

Kalimnos - Khor Fakkan
Kiaochow - Kyrgyzstan

Kalimnos - Khor Fakkan

Kalimnos
Kamerun
Kampuchea
Kandia
Karikal
Karki
Karolinen
Karpathos
Kashmir
Kasos
Kastellorizon
Katanga
Kathiri State of Seiyun
Kavalla
Kavalla (Greek Occupation)
Kazakhstan
Kedah
Kefalonia
Kelantan
Kelantan (Japanese Occupation)
Kenya
Kenya & Uganda
Kenya Uganda & Tanganyika
Kenya Uganda Tanganyika & Zanzibar
Kenya Uganda & Tanzania
Kenya Uganda & Tanzania (Combined Issues)
Kerguelen
Kerrasunde (Russian Post Office)
Khalki
Khania (Italian Post Office)
Khios
Khmer Republic
Khor Fakkan

Kiaochow - Kyrgyzstan

Kiaochow
Kiautschou
King Edward VII Land
Kingdom of Serbs Croats & Slovenes
Kingdom of Yugoslavia
Kionga
Kirgizistan
Kiribati
Kirin & Heilungkiang
Kishangarh
Kithyra
Klaipeda
Kolchak Government (Siberia)
Komotini
Korce (Koritza)
Korea (Empire)
Korea (Indian Custodian Forces)
Korea (Japanese Post Offices)
Koritza
Korytza
Kos
Kotor
Kouang-Tcheou
Krk
Kuban Territory
K-u-K Feldpost
Kupa
Kuwait
Kwangchow
Kwangtung (Japanese Occupation)
Kwidzyn
Kyrgyzstan