I came across this sentence while reading this book and I have my doubts regarding 下すった. Does it come from the verb 下す? Personally, I think it does, but is 下した not the past of 下す? Why 下すった？
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It's a variant of くださった in nonstandard Tokyo dialect (江戸弁).
Few people speak like this in reality today, but I think this is common in samurai dramas, rakugo, and novels from the Meiji era to the early Showa era.
くだす is not relevant; the past form of くだす is くだした.