What Are These Greek Overprints?

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I have a few of this type of overprint on a variety of Greek stamps, but I cannot find what it is actually for, why it was applied, when, or even if it is official. The only ones I can find are unauthorised overprints which used stamps that already had these overprints that were made in the Dodecanese islands, which feature the Greek letters "SDD". The overprints I have are all red or black.

They are definitive stamps from 1946-7 known as the "chains" surcharges. Quite common and I could have sworn they are in Scott.

On an online catalogue which I apparently cannot name, which features stamps from various catalogues, it only has the Dodecanese ones, which are described as being unauthorised (surprising it has unauthorised ones, but not this). I have repeatedly reverse image searched these overprints, and even Google's AI thinks it is the Dodecanese overprint, but it isn't, since it does not have the SDD (it comes to this conclusion because of the fact the only online results are the ones with the SDD). Searching for "Greek chains overprint stamp" or similar, as I had done before making this post, gives me absolutely nothing. I know they are common, I have a few, and I see them in Greek stamp lots on ebay all the time, but it is so weird to me that these common stamps seem to yield no results or information online anywhere.

Wiki mentions them but you probably need to find a Greek collectors forum for more detailed info.

Scott lists them as 472-81 and 501-5 with your stamp above being Scott 503.

The stamp, which depicts the Meteora Monastery, was issued in 1942 and surcharged in 1947. The imprint at the bottom identifies the printer Aspioti-Elka. Its catalogue number in Stanley Gibbons is 622 and its commercial value is approximately zero.

These surcharges were originally intended to revalue stamps after post-war currency reform - e.g. 100d on 1,000,000d - but this and the 10d of the same series are surcharged with the original value. I suspect (but can't prove) that the surcharge was necessary because the original stamps had been invalidated.

That answers my question of "Why surcharge a 50 Drachmai stamp with 50 Drachmai" before I even asked it. I did guess it had to do with post ww2 currency changes.