Thank you, Sabine, for keeping the topic going.
Although I have not found the original skin yet, I have found two earlier skins with the same cigar-shaped rows of buttons: Jethro Amp
. In Kurban, the rows of buttons (and the whole skin) are recolored to brown.
Jethro Amp is definitely a derivative of some earlier skin because its main window is out of style from the rest. So we are looking for a skin which was created earlier than November, 1999. (The latest file in Jethamp.wsz is dated November 1, 1999.)