What I'm using is the old-fashioned, traditional Sodium thiosulphate. Whether it's anhydrous or not - you've got me there! It's the only type I've ever come across - transparent crystals, elongated with several sides and up to a quarter inch in length, about an eighth diameter. My guess is that it's not anhydrous.
You are correct, that's the pentahydrate Na2S2O3 * 5 H2O
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