WW2 Southern Rhodesia Passed By Censor mark.

Hello
I have just got this cover and I have not seen or can found out about this passed by censor mark. The other Southern Rhodesia items I have are all from Civil Internment Camps. Any help at all about this censor mark would be good. Thank you in Advance.
Thanks
Chris S-Weddell.
I have just got this cover and I have not seen or can found out about this passed by censor mark. The other Southern Rhodesia items I have are all from Civil Internment Camps. Any help at all about this censor mark would be good. Thank you in Advance.
Thanks
Chris S-Weddell.