This cover arrived in the post for me this morning :

I have never really got into postal history but would like to add covers to my Union of South Africa collection.
The international postal rate for this period 1919 - 2 1/2d for the first 1/2 oz and then from 1920 it was 2 1/2d for the first 1 oz.
Why was there a 12c postage due charged? Was 1/2d worth 12c in 1920? I take it that there are two separate stamps for the due because they only had the make up values rather than every value.
Any help for this novice cover collector

Closeup of the Due Markings :

Drew