I also agree that it is highly unlikely that a Sindi helmet from the 18th century AD is descended from a 5th century BC Greek Corinthian helmet.
I think it is possible that the masked/visored helmets worn in Iran, Russia and Central Asia are descended from Hellenistic 'Masked helmets' and Roman so-called ' cavalry sports' helmets.
I think a more likely origin of the Sindi helmets is local. I vaguely remember that some Indian
kulah khuds had a very large lower end to the sliding nasal, I suppose it is possible that this expanded nasal could have evolved into the face mask on Sindi helmets. Helmets covering the face are not new to Islamic armour however, 14th century Iranian and Turkish helmets had mail camails that completely covered the face, as did 15th century Turkish and Iranian 'Turban helmets'.
Please excuse the quality of this picture, its a poor photocopy of a black and white photo in a very old book.
'Turban Helm'
