serial number for NX kit lens
I was going to say "There isn't one", having spent some time looking at the 20-50mm that came with my NX100. It is extremely difficult to see one and only about half of the lenses I've got (from Pentax/Sigma/Tamron) have serial numbers on them.
However, if the same as mine - you will see the "CE" mark between the Zoom and Focus rings. When reading the CE mark in the correct direction and in very faint (grey or green?) letters there is what looks like a serial number to the left of the CE mark.
I couldn't see it until I tilted the lens to make the surface underneath the lettering shiny.
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