There are outfits here in the US that rebuild brake boosters, try contacting these guys https://harmonclassicbrakes.com/ They may be familiar with or know a cross reference, it looks very much like same era Ferrari, maybe Dino booster.
PANTERA INTERNATIONAL
A DE TOMASO CAR CLUB
Presents the De Tomaso Forums
The On-Line Meeting Place for De Tomaso Owners and Enthusiasts From Around the World
Clicking on the banner will take you to the sponsor's website.