Interesting Cover, Need Help

This would have been posted during the UDI (Unilateral Declaration of Independence) period, when the British Post Office refused to accept Rhodesian stamps for payment of postage, and treated such covers as unfranked. Perhaps this had something to do with the removal and and covering up of the stamps.

Here one another interesting thing is that they have made one franking over the stamp (marked 1 in red colour), after that they have struck the white paper and made another franking (marked 2 in green colour).

ie Each franking is a set of a round and square , with round part shows the place Coventry Warwickshire and the square part shows the value and the maker of franking machine "PB" which denotes Pitney Bowes.

This cover/card is listed on ebay, this does not have a stamp, is it a postage due , or excempted from postage.. any idea, what this German cancellation means.