Second model in the family of Mills grenades.
Main difference was the adaption of a base plug which would accept a steel rod to launch the grenade from a rifle.
Great variety of plugs known.

The Mk2 type got a larger filler hole.
The Mk3 type had a new design of "shoulders" and a partially re-designed striker mechanism.

Photos ©

More photos on the No.23 RIFLE grenade conversion.