I'd love to see them argue what "conspicuously" means.
I just started SBS'ing a Remington Model 11, and I am planning on laser engraving it inside the receiver, visible when you pull the bolt back.
What is the difference between pulling back a bolt versus pulling off a scope mount, as if you were needing to see the SN engraving on top of an HK MP5? There are no rules or clarifications from the ATF, AFAIK, about being prohibited from covering up a SN (or any required engraving/markings) with a scope mount.