Although I was confident these foundations were for a shower block, it took me awhile to find the official abbreviation for SALs, which is what the army sometimes wrote on their maps and plans around the World War 2 era. Perhaps some sections of the army or the community still use this abbreviation.

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SAL - Showers and Latrines

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Apparently it also stands for Shit At Life, a nickname given to a soldier who fails at everything. But the abbreviation as Showers and Latrines is said to just be something they say in the older days.