Koen,

If you look at the top head lug in your photo closely you can see that the lug line encircles the top of the head tube and then drops down and then comes to a point just above the top of the head badge and then swoops up and back to the underneath of the top tube. The point is what is sometimes referred to as the "ears". Later bikes don't have these "ears".

Yes, I would say your bike is late 50's but I don't know exactly when they changed to smooth lugs/flat crowns.

Cinelli dating is one of the "dark arts".

thirst