If you want to learn more of the Double Heads than probably the best resource is the Rhodesia Study Circle, and, as Bobby has already pointed out Stanley Gibbons will also be very helpful, in particular, the "Southern and Central Africa Catalogue".
There are an enormous number of varieties, SG lists about 10 variations of the 1d alone. To narrow down what you have, first, check the perf count.
Here is the Rhodesian Study Circle page on the 1d Double Head -
http://www.rhodesianstudycircle.org...tish-empire/Clive