- document
- de Ruiter, H.J. (author) report 1967
- document
- de Ruiter, H.J. (author) report 1964
- document
- de Ruiter, H.J. (author) report 1964
- document
- de Does, J. Ch. (author), de Ruiter, H.J. (author) report 1962
- document
- de Ruiter, H.J. (author) report 1962