Cover of the day - On Servjce

[Ross Debenham]

I attach two scans of a "On Service" cover from the Staff Officer, Premier Camp, South Africa to United States Naval Academy, Maryland, USA. Hit with Commanding Officer, Premier Camp cachet. Also hit with Ovongo Beach. Sent via civilian mail and censored in Durban. What I am inquiring about is the "On Service" on the top of the cover which also appears on other covers sent from South Africa. Is this a "legal" requirement as opposed to "On Active Service" covers. I also just acquired a typed censored cover from East African Army Post Office 60, sent to South Africa with "On Service" typed on the top of the cover. Can anybody enlighten me.