Ariel, You're right. Not a Russian painting. It is Italian, mid 15th Century by Sano di Pietro (Siena, Italy 1405-1481): Beheading of St John the Baptist.
Here's another of his works showing straight swords, he certainly knew how to draw a straight line
The rest would be speculation. However, even though the time period matches, I must agree with you, Marius, Motan and Dave that it is quite unlikely that recurved sword on the first painting refers to recurved sabers of Russland origin. Thank you for an interesting post and discussion.