When you say "fitted with" do you mean as delivered new, or installed by or at the request of the owner? My first car was a 1968 Mini (848 cc), and certainly it had inertia-reel belts, but I don't remember whether they were standard, an optional extra, or whether I had them installed before delivery (like the ammeter which I certainly did have installed).
Ken G, 1925 Rover 16/50 (San Francisco)