This type of bag was called a TAP bag, and was copied from the French (TAP is short for Troupes Aeroportés).
It was introduced in the early 60's in the belgian army for use by the para-commando regiment,
replacing the british style small pack.
It was first used by the para-commando regiment in the 1964 operations in the Congo (Operations Red and Black Dragon).
This type of bag later became part of the Model 71 individual equipment, so became general issue when this equipment was introduced
in the late 70's. The bag was then no longer made in canvas, but in the plastic type of fabric so typical of the M71 equipment.