this is not a Panerai thing and Patek has done it before
In 2006 when they opened the new Patek saloon in Geneva they made 2 special watches
One was the 5105, a Platinum Gondolo which also had a 'vintage' movement, the 9-90
This reference was limited to 100 pce
The other was a stainless steel Calatrava 5565
I think the last one was limited to 300 pce