A member of the UK Jamma Forum told me that the yellow start buttons on my european Robotron are original.
In fact I have NEVER seen any picture of the Robotron panel with those yellow buttons. Always the blue translucent ones. I ripped the panel for restoration and found two light bulbs in their sockets and there are two wires for lighting them (white and white/yellow). So I think that those blue buttons have originally been in the european Robotron,too.
Or does anybody have any pictures with the same buttons like mine???
http://www.thludwig.de/pics/arcade/robotron/robotron_panel_1.jpg