The model 520NN was produced from 1957 to 1963 c.ca and it was typical of the Italian market.
The overlay for the 500 line was probably manufactured in Italy, although it is not known precisely where ad by which factory.
This pen is in very good condition withontly some minimal signs of use.
The overlay has a couple of light marks ( which are not even dings) and the pen is priced accordingly.
Nib grade is M