27 CFR 478.92(a)
This identifies where and what manufacturers and importers are required to imprint upon the weapon. Barrels are allowed for makers marks and origination, NOT for SN's.
I can't say that I wasn't given bad information, or that the laws have not changed, but I talked to the BATFE about 15yrs ago regarding this topic - older Marlin's had the SN's down the top tang, so I had to move them to the side of the receiver to install tang mounted peep sights. At that time, I only had an FFL, not manufacturers FFL, and the agent I spoke with approved that migration, sufficing that I kept documented record of the movement and that the SN remained unchanged.
It's correct that you're not able to move the SN on a Ruger SA, excluding the Bearcat, to the grip, as it's not part of the receiver. The SRH, GP100, and SP101 also cannot be so marked, because the "spur" or "tennon" of the grip is not conspicuous on these models. The Redhawk and the Bearcat are the only current production Ruger revolver models that can legally have the SN on the gripframe.
Many states, if not the fed, have laws against altering the makers marks, model numbers, or SN's for firearms (google search "altering firearm ID mark" CT, OH, KS, are the top 3 or 4 on the list).
A holder of a manufacturer's FFL can obliterate any marking on any firearm (Non-NFA), so long as they replace the markings with their own in kind - hence how guys like Al Harton, Bowen, etc can do it. Manufacturer's FFL isn't any more difficult to get, nor really substantially more expensive than that of a "gun seller" or "gunsmith" FFL. You'd actually be surprised how easy it is to get an NFA-makers license (albeit I have noticed a black helicopter circling my house ever since I submitted my application).