Text and picture copied from "Japanese Infantry Weapons" cincpac cincpoa Bulletin 55-45, 15 March 1945 :

"A new grenade, equipped with a pull-type igniter, has been encountered. Its design makes it readily adaptable as a booby trap. Unlike the Type 91 and 97, this grenade has five horizontal and no vertical serrations. Fitted to the top and bottom are two rings probably used for anchoring when the grenade is used as a booby trap. There is a lead cover release grooved to provide a grip for the fingers".