I know 100% I successfully transferred coins between the wallets over a very short period of a week or 2 max and then never touched the wallet or Bitcoin again until 2016 or so. By then my PC were long broken and got rid off.
Did you also get rid of HDD of that broken PC?
Does anyone have an idea what wallet type that had the same look and feel as Electrum existed in or before 2011?
There has to be an answer to this mystery. Let's solve it.
I only can think MultiBit,
https://en.bitcoin.it/wiki/MultiBit. But i don't remember the original version generate 12 seed/mnemonic words.