You know, the country codes which people put on the back of their cars when they drive abroad and which also form the basis of top-level country domains. South Africa is ZA rather than the obvious SA.
My first intuitive guess was that it’s based on the name of the country in Afrikaans rather than English, but no - the Afrikaans name is Suid-Afrika.
SA is the postal code for Saudi Arabia.
In Afrikaans, South Africa is Suid-Afrika, but in Dutch I believe it is Zuid-Afrika. That may explain where the Z comes from, anyway.
(Isn’t Saudi Arabia KSA on car stickers, though? They often don’t follow the two-letter codes.)
Edit, no, it seems Saudi is SA. I have seen KSA though, unofficially. http://users.telenet.be/worldstandards/carcodes.htm
Have you noticed how radio call signs are often the same as the national code things? “HP” for Paraguay which (used to) us radio station call signs, and “HZ” for Saudi Arabia, which doesn’t.
I don’t know about car stickers, but the convention for postal codes calls for two letters. Here’s a page I use all the time.
The car stickers are a real mishmash of one, two and three letters - I added this link above http://users.telenet.be/worldstandards/carcodes.htm
E.g. in Europe we have the commonly seen F for France, D for Germany, A for Austria etc, then there’s GB for Great Britain (although it should arguably be UK), NL for Netherlands, DK for Denmark and so on, and then three-letter ones such as FIN for Finland, BIH for Bosnia and GBG for Guernsey.
There’s no rhyme or reason to it that I can see, e.g. why does Mexico use MEX on cars when MX would be perfectly unambiguous? Why QA for Qatar instead of Q? There’s no other country starting with that letter!
Colophon is correct: the name in Dutch is “Zuid-Afrika”. “ZA” was adopted as an international vehicle registration code in 1936, at a time in which the position of Afrikaans versus Dutch as an official South African language was still in flux. (When South Africa was created in 1910 the official languages were English and Dutch; in 1925 the reference to Dutch in the South Africa Act was declared to include Afrikaans; in the new republican constitution of 1961 Dutch was replaced by Afrikaans.)
The “SA” code was unavailable not because of Saudi Arabia, but because of the Saarland, which was a League of Nations mandate territory from 1920 to 1935, when it was absorbed by Nazi Germany. Presumably it was felt that the code could not be usurped by South Africa so soon afterwards.
As has been pointed out, the international vehicle registration codes are not the same as the two-letter codes used for top level domain names. Those are based on the standard known as ISO 3166-1 alpha-2. I suppose when that was created “ZA” was seen as an easy way to avoid a conflict with Saudi Arabia.
I think HZ may be for “Hejaz”, the Western region of Saudi Arabia.
Finland used to be SF.
This has long been a topic of interest to me (country codes) - I’d say I’m the only Doper that has the entire ISO 3166-1 alpha-2 list (the basis of TLDs) memorized, but, knowing this crowd, I’m probably not alone.
That being said, I know that the indeterminately reserved list of 3166-1 alpha-2 codes are because of different road vehicle country codes in the UN conventions on road traffic - I’m wondering why those aren’t being used. Or, to standardize things (the “S” in ISO 3166-1 alpha-2 stands for “Standardization” after all), why can’t vehicle codes just use ISO 3166-1 alpha-2? While I’m one of the only freaks that has the entire list memorized, I’m sure most people at least know their own code due to the list being used for Internet ccTLDs (with the exception of .uk for the United Kingdom, whose ISO 3166-1 alpha-2 code is GB). But then I believe the codes currently in use have been in place since the dawn of the automobile, so could be hard to replace.
That pretty much includes everything I wanted to know, particularly the follow-up question (to the statement that South Afrika is Zuid-Afrika in Dutch) why the heck the name of the country in Dutch, rather than the very similar Afrikaans, was chosen. Thanks!
Suomi-Finland UIVMM.
And I believe Switzerland used to be CH (Canton Helvetica?).
Pleasure!
Switzerland still is CH, which stands for Confoederatio Helvetica (“Swiss Confederation”).
♪♫ I left my heart in Suomen Taasavalta ♪♫ 
That’s right. They chose the Latin version of it because, as a multilingual country, they didn’t want to give preference to any particular language.
If you want strange, non standard, codes you should take up cataloguing. Instead of using standard codes the Library of Congress made up their own and has managed to export them all over the World (together with their bibliographic format).
The “standard” being Dewey?
The standard being the aforementioned ISO 3166-1. Classification and cataloguing are two totally different things.
What’s interesting is that, if you’re a military historian with an interest in the period, you might come across mentions of the Zuid-Afrikaansche Republiek (ZAR), sometimes known as “The Transvaal Republic”- one of the Boer Republics involved in the Boer War and later incorporated into the Cape Colonies after the war’s conclusion.
So as a Military Historian, “ZA” for “South Africa” makes sense, but since most people have better things to do than remember the names of short-lived 19th century republics, it does seem an odd choice for the most part. 