I believe I had a prototype Digital Audio G4/533 at one time, years ago. It had the silver metallic "Apple Development Team" inventory sticker on the top surface of the case.

I bought it at a bargain price from a reseller in Austin in '04. It did seem to have different markings than production motherboards, but I never took a picture. I ended up selling it to a guy who turned it into a nice budget audio workstation.