I'm thinking that just the L model had the retractable seat belts into the rocker panel area.
Probably just another safety feature like the bigger bumpers
Cool pushbutton site with lots of info
http://www.smartstoreinc.com/pantera/index.html
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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.