The serial number is a Criminal Code requirement. Has been for decades. The UN marking business was to add a country code to the serial number. When every step in the production run costs money, pulling a block aside for another step adds to the overall cost of things. Think about the last time you did a home-style production run of anything being done sequentially. If you realized something should be sanded or a screw turned, then put those steps into context when a thousand guns have to be treated differently if the plant is pushing through ten thousand. Ten percent might not seem difficult, but that means no mixing in or replacements if QA/QC finds flaws. No adding to the order if the market looks better than when the run was started. No cancellations if things go sour either.