depends on who painted the copy and how they were deemed at the time of being a copy or a close replication. I agree that even the original painting may be missing things that we in the photo age take for granted. I think general style and colourations would be good but whether the item in the painting had x style of pleats etc, weelllll that would be harder. Due to the fact that the originals were destroyed during the war, there is no chance to take the copy to the original and say....welll that's off etc. Who is saying that the original was actually better. Would take it as secondary documentation for sure as there is plenty out there that you can take as 1st. So utilize for supporting docs. Like I do of the Vatican wall painting. have no clue when it was painted, but darn, looks close to what I am studying. So supporting documentation. And thas my 2 dutch pennies as regional luv