I agree the buttplates are original, as in manufactured during the same period as the guns.
Does it make sense however, that Ithaca had proprietary buttplates for Crass, Lewis and Minier doubles
and LAC has a proprietary buttplate
And during the same period Hunter Arms was offering "The L.C. Smith Trap Gun" plates
And they were in competition, but all 3 offered that same buttplate??
A.H. Fox BTW had a simple grooved buttplate without the Fox name, and the plates have been found on Fox guns also.
We won't KNOW until we find a catalog showing the buttplate in question. I sure wish the makers name was on the wood side