From the Illustrated Parts Catalogue for CVR(T), 1095-99-820-5398 is Cap, Discharger, Smoke Grenade; Rubber, 3in OD x 0.060in Nom Wall Thick. Note that this is for a No.12 Mk 2 launcher, but there also seem to be Mk 1s mentioned. Perhaps both types have been fitted but the cap only fits the Mk 2?
Andy