The “S” letter in the logo of our DIS Workshop is a stylised shape of a Warsaw mermaid (in Polish “syrenka”) which is the capital’s coat of arms. Polish syrenka is cognate with siren, but she is more properly a fresh-water mermaid called “Melusina”. This imagery has been in use since at least the mid-14th century. The syrenka has traditionally held a silver sword in her right hand (source: Wikipedia).
In our logo, a parity operator has put the sword into syrenka’s left hand.