The Belgian Energa Grenade was first developed for the SAFN 49, hence the use of .30/06 Launcher Cartridges; The British adopted it and its 22mm Launcher Tube for the Rifle No.4, hence the .303 Cartridge;
The Belgian Police also had .303 cal. Rifles (Both No.4 and No.1, obviously the Laucher was fitted to No.4 Rifles), and the FN-FAL, itself first in general issue in 1953-54, then had the Energa attachment and a 7,62x51 Cartridge supplied.
The 7,62 Launch Cartridge was usually supplied (one of) in a Plastic Plug fitted in the tail-fin tube assembly ( From an Italian version of the Energa grenade).
I fired quite a few Black (inert practice) Energa grenades whilst in Infantry in the late 1960s (from L1A1 Rifle.).