Anyone ever see these gold hallmarks?

[myrongaines]

I was sold these post backs as solid 18K yellow gold. The hallmarks on the back are ones I haven't seen before. It's stamped CU 8K. Is it safe to assume it's really just 8K yellow gold instead?

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[pearlsngems]

I have not see that specific mark but I'd assume they are indeed 8 karat gold.

 

[kenny]

On the Periodic Table of Elements Cu is the symbol for copper.
Maybe it just indicates there is copper in the alloy.

So, maybe it's not a mark indicating the percentage of gold content after all.