Hi and welcome to the forum!
I would suspect the helmet is Hungarian issue given the location where it was bought from?
The are several detail differences between it and a true Soviet M40.
Differences I see:
shape of the shell (look at the front lip)
chinstrap and chinstrap D rings
Liner pads in fabric, not artificial leather
it's look like late war type of M-40 soviet helmet, in shape, but not original paint, star, liner and chinstrape.
It's definately not a Soviet by the shape. The Hungary and Polish made helmets are more round. The Soviet is more than the egg.
The helmet shown is Hungary made one
my Skype: warrelics
The helmet posted is a post war Hungarian helmet. As stated by Ade and Dimas, the liner materials, chinstrap and shape are typical for this type... the dark green paint and small red star are also typical of the Hungarian helmets.