GB With Pm Covering I Think Colony Name.

Have this GB that sufferers the stamp rule that a PM will invariably cover the most important part needed to identify. I know I'll feel foolish when I get an answer but I've searched every colony I know of. The postmark looks to be Johannesburg but it is not listed in any South African GB controlled state. Since it is a postage & revenue I assume it should be a listed stamp. Can someone please tell me where it belongs in the GB regime. I use Scott.

Transvaal.

Thank you so much.