I think this is Marklin plus,
a building system by Marklin wich ran from 1973 till the end of the seventies

also see http://www.metallbaukasten.de/marklin/plus.html

thanks to the comment of Jan Windemunt for pointing me to Marklin

I think this is Marklin plus,
a building system by Marklin which ran from 1973 till the end of the seventies

also see http://www.metallbaukasten.de/marklin/plus.html

thanks to the comment of Jan Windemuth for pointing me to Marklin