To me it sounds like you have the serial number and maker mark are two different components.
The serial on the trunnion which RIA used from a SACO demil and their own makers mark on the channel. Most RIA guns have the serial and maker markings on the channel (mine does anyway) as RIA transitioned to making their own trunnions in house at some point. I presume after they ran out of clean Saco demils to use.
This is where the complexity comes in regards to replacing the sheet metal.
26 USC 5842 - Sec. 5842 states the following.
(a) Identification of firearms other than destructive devices
Each manufacturer and importer and anyone making a firearm shall identify each firearm, other than a destructive device, manufactured, imported, or made by a serial number which may not be readily removed, obliterated, or altered, the name of the manufacturer, importer, or maker,and such other identification as the Secretary may by regulations prescribe.
26 USC 5861 Sec. 5861 states the following.
§5861. Prohibited acts
It shall be unlawful for any person—
(g) to obliterate, remove, change, or alter the serial number or other identification of a firearm required by this chapter; or
(h) to receive or possess a firearm having the serial number or other identification required by this chapter obliterated, removed, changed, or altered; or
Title 1/Non-NFA firearms can have any marking removed or altered except the serial. Title II it doesnt appear you really can’t mess with any of the markings defined by regulations prescribed by the Secretary.
In this case I don’t believe it would be legal to remove the maker marking on an NFA firearm as they are defined in Tres Sec regulation 479.102... of which USC Treasury Code 26 part 5861 states that these markings "as defined by treasury regulations" cannot be obliterated or removed.
I am not sure how this would play out if the maker marking is on an easily removable part like a quick change barrel, where regulations do technically allows all other markings besides serial (which must be on the receiver) to be engraved. However, there was recently an article in SAR by Mark Barnes about folks who messed with the original model markings on transferable HK machineguns (HK94 to MP5) and made their guns contraband as a result.
I would be pretty leary about removing the original manufacturer markings (the RIA stamp on the channel) on a 30K transferable machinegun. I beleive alot of folks wouldn't consider purchasing (unless the price was really discounted) a M60 that had a huge section of its receiver replaced by the previous owner via a rivet tack weld operation and of which some of the original manufacturer markings were lost in the process.
Given what you have at stake, (i.e a $30,000+) gun of which legal and build pedigree is of upmost importance to future buyers this is what I would do if I were you assuming you ever wish to sell it….
I know people say never write letters to the ATF, but in this case I would get approval in writing from the ATF on exactly what you want to do. i.e. replace the damaged sheet metal channel on a RIA M60 and make sure it is legal to lose and/or remark the original manufacturer mark onto the new channel.
I would then either ship the gun USPS Registered mail to Desset Ord and insure for the max 25K liability they allow. Alternatively, you could book a direct flight and personally deliver it to Dessert Ord for the rework and then go back to pick it up when it was done.
Good luck with whatever your decision is and I hope it all turns out for the best.