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It looks like either pushing in on the center section, or possibly rotating the center section, but there's no practical means for either...

It looks like there's the possibility of putting a small wire into the smaller hole on top of the barrel that forces the cam lobe looking deal inward to pull the sear out. I'm thinking that's probably the most likely. Basically ends up being a small button above the barrel and pushing the button will fire it (or given the shape, pushing the whole thing against something would be more likely).

It also looks like a quarter turn of the upper section in relation to the bottom could possibly disable/enable it. I think that's the purpose of that lobed section...