It looks like there's a parting line around the edge. I bet the inner plate rotates slightly to unlock it. There's at least 2 but probably 3 holes which may locate springy bits to stop it coming unlocked.
Excellent use of the 'Enhance' function, Officer Deckard.
@mespilus started
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It looks like there's a parting line around the edge. I bet the inner plate rotates slightly to unlock it. There's at least 2 but probably 3 holes which may locate springy bits to stop it coming unlocked.