The use of a grenade to signify substantive Grenadier Guards Corporals seems to have continued until after WW2. See enclosed photo of a GG guard of honour for Princess Elizabeth. In the second rank can be seen a Corporal with grenade above his stripes. The photo can be enlarged at the Wiki link:
https://en.wikipedia.org/wiki/5th_Gu...moured_Brigade
Also right hand man, centre rank, in the second photo. Enlargement here:
https://www.rct.uk/sites/default/fil...1353339573.jpg
I suspect the use of the grenade above two-stripes ended when the Lance Sergeant rank was abolished in the rest of the Army and the Foot Guards decided to retain it, but doing away with one of the two lowermost grades to compensate.