Because it isn't, and has never been, "S-tank". As noted by Gryphon above, it was ALTERNATIVE S, for Swedish, that is to say a domestic tank in the procurement "competition". If the 103 is supposed to be the "S-tank", then the M60 Patton and Leopard 1 are "A-tank" and "T-tank", respectively, and if you try to argue THAT with anyone with any knowledge of tanks, they'll laugh at you.

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Stoke Mandeville, Esq & The Victorian Ballsmiths
"Nobody Want Verdigris-Covered Balls!"