The M18 is a thin steel smoke grenade, filled with a HC-mixture.

Four smoke emitting holes are located on the top and one on the base. Variations on this are known.
Early models were painted light grey, later models were painted olive drab.
Fuzes M200A1, M201 or M201A1 were fitted on this grenade.

Description copied from FM 23-30-1944 :

This grenade is similar in size and shape to the irritant gas hand grenade CN-DM M6. It is equipped with the M200A1 igniting fuze with 2-second delay. Seven colors of smoke (red, orange, blue, green, black, violet, and yellow) are available. The mixture in the M18 grenade burns for 1 minute and in the M16 (limited standard) grenade the mixture burns for 2½ minutes.

Photos below © Jean-Michel Roth.

Old ww2 version:

Photos Geert

Data for the grrenade shown above:

Empty weight with fuze : 177 gr
Heigth w/fuze M201 : 145mm
Height w/o fuze : 118mm
Diameter : 60mm
Empty weight with fuze : 177 gr

Other colours. Photos Craig.