"...I believe it operated using special blanks (ie non projectile rounds) and a grenade was placed in it and the pin pulled (the cup keeping the handle from springing up) then the butt placed on the ground with the rifle at about 45degrees the vent knob adjusted for range and the trigger pulled. The wire wrapping on my SMLE is apparently to strenghten/stiffen the wood furniture..."
Just acouple of additions (from a faulty memory)
The grenade was fitted with a metal plate (supplied in the grenade case IIRC) screwed to the base plug to act as a 'sabot'.
The ballistite cartridge appears like a standard blank but ( believe) with the 'projectile' end painted black and is loaded with a more efficient projecting charge than the black powder used in standard blanks.
The aim of the wire wrapping was to prevent the fore-end stock from splitting in use, this having been brought in in the earller period when the dischargers were generally 'shot-out' rifles and the grenades were of the 'rodded' variety, with the greater possibility of barrel damage/bursts.
Tom the Walrus