A symmetrical variation of the "violin" accordion registration symbol, with two "middle" (8') reed ranks. Strictly accurate for sharp and flat detuned ranks (as in a musette accordion with three "middle" ranks) but also used on some accordions where one of the ranks isn't detuned (and would be represented by a centered dot if alone).
https://upload.wikimedia.org/wikipedia/commons/archive/c/cb/20081006090237%21Accordionstops_violin.svg
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