Doubt this is still available but this is the way to go. I wouldn't think the PermaCap was custom made for Hall. Quick search on google didn't turn up anything with inside threads like this, so maybe they were. The only markings on mine are: Made in Germany, CARB # D-220 and a patent number.
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.