It can be anywhere on the firearm (lower rec, upper rec, barrel), though putting it on the lower is probably best for a number of reasons, mainly because if it's not put on the lower, then you'd have to mark EVERY short upper rec/barrel you wanted to use with your SBR.
The markings should be in plain sight, meaning they can be seen without disassembling the rifle at all, or at least without any disassembly that requires tools.
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