Okay... I am going out, way out on a limb here... not having an actual clue as to the use of Silk Velvet ( yes they had it) for making a woman's gown... LATE 18th century as in American Rev. War... velvet is a MAN's fabric and was not used in women's dresses except MAYBE as a trim, or in a masculine styled riding habit.. I have no evidence if this was true during the GAoP, but I have not come across it in women's dresses as of yet...
and most sorry to disagree with you Cheeky, but all the real silk velvets I have seen and handled are really quite shiny (not flat like cotton velvets)... as matter of fact rayon velvet pretty much passes in appearance to a good silk velvet... and if I recall a conversation with Kass once correctly, no one is truly manufacturing 100% silk velvet now a days, tis always a blend of some kind.. The black silk velvet I have on the cuffs of the brown coat I have had for ...ahem... decades.... now I feel old...**sigh**