The FCC documentation (pictures of the WRT1900AC-V2) revealed that at least that WRT1900AC-V2 was using a Marvell "88F6820-A0 C160" CPU, mind you, that's the 1.6Ghz variant.
(FCC documents regarding WRT1900ACV2 can be found here:
https://fcc.io/Q87/-WRT1900ACV2 )
The FCC-ID of the WRT1900AC-V2 is: "Q87-WRT1900ACV2" and the FCC ID of the WRT1900ACS is also " Q87-WRT1900ACV2".
The WRT1900ACS was never certified as a different model / hardware-version but as the WRT1900ACV2.
So both routers must be identical, otherwise they would have to get the new hardware certified and it would have gotten a different FCC-ID.