"Turning his little savingsbox about in his hand," Mr. Deasy tells Stephen, "These are handy things to have....You just buy one of these machines. You'll find them very handy." Trolley conductors of the time made change with barrel-and-lever devices that could be clipped to a belt, but they could hardly be called boxes or turned about in one's hand. Joyce is more likely referring to a device that could be carried in a pocket, such as the one shown in the first two photographs here.

JH 2021

Spring-loaded pocket coin holder, made pre-1938. Source:

Side view of the same device. Source: