It depends on which of the Cyclo Benelux Mark 7's you have. They came in 3sp 1/8", 4sp 1/8". 4 sp 3/32" and 5 speed 3/32".
the sprocket range was 13T-24T and with a double chainwheel total capacity was 15T.
The RD was not designed for the capacity you are expecting of it.
see Cyclo - Benelux Mark 7 Gear scan 2