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This is an Activator, basically the on-off switch for those of us wielding a lightsaber on days where the force isn't so strong with us. (Perhaps there is a bad motivator somewhere.) This part, like my other modular lightsaber parts, is sized to accept a small flashlight inside like the one found here: https://www.harborfreight.com/3-12-in-Mini-Flashlight-69111.html
This Activator is non-functional. That is to say, it has no moving parts.