Objects like this were made by the million, with the sole purpose of destruction. It weighs just over one pound and fits comfortably into the hand. It is easy to imagine throwing it.
This is an incomplete case, yet to be finished by drilling holes to take the firing mechanism and of course it is innocent of any trace of explosive. When complete the safety clip held a lever in place against the side of the case; to use it all that was necessary was to hold the lever firmly in place, pull out the safety clip and throw the grenade as far as possible before taking cover from the explosion. The grooves in the case allowed it to break up in the explosion, increasing the destructive power.
This one belonged to my father who was born in 1913, he never told me how he got it. Because it is unfinished my assumption is that a relative worked in a munitions factory and brought it home as a macabre souvenir, perhaps at the end of the war when the factory closed.