yeah i saw that too , pretty sure its a one-design.
I had a plain white imco that was an early powerbox model and I think came just before they did the board with the Atlanta '96 graphic , I reckon that is the same board and likely has a one design sticker on the other rail not shown.
the ugly yellow is most likely a backyard sugar re-grip .
not sure about the Concept label that some Mistrals had mid 1990's , I think it refers to their M-tech construction that Mistral say is a sandwich construction with ASA skin.
there's a Screamer with the Concept label here ;
www.seabreeze.com.au/Classifieds/Windsurfing-Boards/~vd6g3/Mistral-Screamer-278-cm-101-litres.aspx?_page=1&search=MsVd7Fn%2F%2FIrz2wJoLe%2BWf8PfthSGSW%2BQ