Mills Bomb is the nickname for a series of British grenades developed during WWI by William Mills. There are several models of the Mills Bomb, each with an official British number designation:
No. 5: The first version of the Mills Bomb introduced in 1915, which can only be used as a hand grenade. Features only one variant, No. 5 Mk. I.
No. 23: Modified so it can be used as a rifle grenade in addition to being a hand grenade. Features a modified base plug drilled with a threaded hole that accepts a short rod, which can then be fired with a cradle-styled launcher mounted on a standard-issue Lee-Enfield. Features three variants: No. 23 Mk. I through III. Mk. II featured an improved iron base plug, while Mk. III featured a modified grenade top design that is kept in the No. 36.
No. 36: Introduced in 1917, the No. 36 was designed to be used with the No.1 Mk.I Cup Discharger when used as a rifle grenade. The base plug was modified again so a gascheck disk can be screwed on to the base plug when the grenade was used with the discharger. The No. 36 featured a No. 36M variant which was coated in shellac for waterproofing, initially designed for the 1917 Mesopotamian campaign. By 1918, the No. 5 and No. 23 Mills Bomb models were declared obsolete, and the No. 36 became the standard British fragmentation grenade, which was later replaced with the No. 36M in 1930s. The No. 36 (and No. 36M) held the position as the standard British frag grenade for over 55 years until 1972.
The specific model and mark of a Mills Bomb are marked on the base plug, but as all base plugs are interchangeable, identification may become muddy.