If anyone advertises a clip-on registered receiver and discusses “sear” as anything other than a clip-on with a modified “sear/trip”, then that person is trying to fool a potential noob buyer into thinking it’s a registered sear. If they list it like, “registered receiver sear gun” or “registered receiver with sear”, odds are, that person is intentionally introducing “sear” into the description just to make you think it’s a sear gun.
I think the issue is you have both an unserialized proprietary “sear” and an unserialized modified metal frame (or pack) to hold the sear. Typically the pack modification is removing the semi selector stop so you can rotate the selector to F/A. So you actually have two unserialized parts in the gun that can be considered machine guns on their own if you start separating things. ATF has tolerated these “collection of parts” conversions, but I doubt they would agree to let you serialize the sear and destroy the other parts (the registered serialized receiver and the unserialized pack.). I have never seen an ATF approval letter for destroying a registered, serialized part. There are definitely people who have destroyed conversion parts and reconfigured their guns, but this has typically been parts that could already be moved from gun to gun.
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Another way to look at it is that the registered machinegun part ALWAYS has a serial number on it.
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There are a number of improperly registered UZIs out there too. (RR guns left in stock semi configuration with blocking bar, with modified bolts that should have been the registered device)
I bet some improperly registered receiver machine guns have been physically "corrected" by capable hands before, but we won't go there.