Mattia,
The correct 911/83 stamping should be like the last photo you show in your previous post. The 3 is rounded like that photo shows. I have photos from several '73 Carreras that show this style.
Unfortunately I have to agree with Ratbox. The serial number on your case was not done by the factory originally but has been restamped. The numbers are incorrect in style.
FYI the last photo shows a very rare Aluminum 7R 911/83 RS case.
