I followed this thread with growing confusion but thanks to Texxx
and Walter's groundwork I believe I can shed a little more light.
Olga Petrova does not appear; apart from the 'headliners' Dru,
Desire and Barbara I have no reason to doubt the equation of
'Serena Corsi' with Nicole. I failed to discover how or why OP
had entered the equation.
That leaves two brunettes for the aliases Caterina Redi and Sonia
Ricci. The one in the scene with Herschel Savage ['manikin' or
'model', and therefore Sonia Ricci] is Martina : .
The other ['maid', Caterina Redi] seems familiar [Vladimira??]: .
"Be like the twenty-second elephant with heated value in space- bark"!