Help me out here though, if the gun is 1926 production and the SN was filed off prior to the '68 law - why would the 1968 SN requirements have any effect? Why wouldn't it essentially be "grandfathered" in?
No, a gun manufactured prior to '68 wasn't required to have a serial number. As such, it would be "grandfathered" and is still legal to own. The law states that any gun with a serial number or other manufacturer's identification that has been obliterated or altered is illegal to posses, period. It couldn't be any other way, if you think about it. How would you prove when
a serial number was obliterated, or, with the number gone, when it was manufactured, for that matter?
As for restoring the original serial number, the only way to sometimes recover an obliterated serial number is to use acid to etch away the metal where the number used to be. The metal under the stamped numbers will be compressed and hardened, so will wear away slower than the surrounding metal, leaving a faint impression of the original number. However, to do so ruins the appearance and hence the collector's value of the gun.