R.S.A. - 4c Dr. Diederichs'
I can't find this variety recorded.
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I have seen these before. I have a few in blocks and or pairs somewhere. I showed them to a dealer who is more interested in stamps of the republican Apartheid era than me. He said that he didn't think that they were worth anything. He did not comment on whether he had seen them before or not nor whether they were recorded. I imagine they must be but where? Certainly not in the SACC 2008/9.
The top value 15c (SACC No. 382) in this issue suffered from over-inking which gave some of the stamps a blackish-blue appearance. This is listed as a variety, No. 382a. Yours is not. The black in your stamp affects all of it except the background. You would have thought that someone would have observed and recorded this 4c variety by now as the stamp was issued in 1975, some 47 years ago!
Stamps of this era showing the old regime, although of great interest to some, suffer from being a bit too recent ie. modern. Nevertheless, your block is attractive and will be desirable to someone. Congrats on the succinct description!