✅ SOLVED Looks Kind of Like a Skeleton Key, But???

[oldmxrat]

It seems to be made of brass, 1.5 inches long, and looks as though the corrugated 'flag' part was supposed to rotate independently.

 

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[Gare]

Just a thought mind you but maybe the key broke off in the lock and they had to take the lock apart to get it open and just threw your part away ?

 

[50bmg]

looks like a double sided key to me, with 1 end still in whats left of a lock tumbler

 

[Hart45]

It seems to be made of brass, 1.5 inches long, and looks as though the corrugated 'flag' part was supposed to rotate independently.
View attachment 2114541View attachment 2114542View attachment 2114543

Keyhole blocker with key broke off in it

 

[oldmxrat]

Keyhole blocker with key broke off in it

That's it, you nailed it!
Found this image online with your description, thanks!