Not sure about mass-produced, in current dollars the price was equivalent of about $8,000, hardly a lens that every amateur was going to jump on.Going by the price, not many remain. Unfortunately, those bits of information are extremely hard to come by outside of Leica.
It was a mass-produced lens, but we don't even know the exact number of 1200mm f/5.6L lenses that were made. Even that one has a large range of estimates.
There are two on ebay for $16,000+
It has in my book the best mount with the Canon 1st generation FD spring-mounted bayonet that moved the ring enough to secure the lens when the lens was pushed into the body, and had zero friction so no wear on the interface.