Any idea?

[Jenjen21]

 

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[Jose The Goon]

To me it looks like what is called a "biscuit" that woodworkers use for joining wood.
They are made of both wood & metal, but mostly wood for woodworking.
Metal ones could be used by welders or other metalworkers, as well as woodworkers.

 

[Digger RJ]

View attachment 2057622

Nice!!! Congrats!!!

 

[pepperj]

Size reference is good.
The other side blank, and how thick?

 

[cudamark]

How big is this thing? One inch long? One foot long? Something for scale might help.