The stamp shows us ALBERT SCHWEITZER https://en.wikipedia.org/wiki/Albert_Schweitzer in a dugout on the Ogooue River in Lambaréné, Gabon.
A dugout canoe is a craft fashioned from a log by hollowing out the centre to make it lighter and more buoyant and to provide a place to sit or stand. The dugout is used in most parts of the world where suitable trees are available. Hew out by an adze, stone, bone or ivory sharped tool or burned out.
Source: Internet and http://www.postzegelontwerpen.nl
Netherland 1975 50c sg?, scott?