Some sort of custom made plug, I had similar things to affix the drum in the washing machine, obviously without the rawl plugs. If you don't know what furniture they go with, this makes them surplus to requirements... metal in metal recycling, plastic in to hard plastics at your local tip!
Just look around, curtain track fixings? blind fixings? some sort of fixing to hold something up?
Either as Nozzle says or put them back in the back and at the back of the drawer again
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