I ordered one for the 1983 240D that I recently purchased that had 199K+ on the clock when I got it. My 1981 300TDT is getting close to the first one as well.
The milestones are:
155,000 miles (250,000 km)
312,500 miles (500,000 km)
625,000 miles (1,000,000 km)
I hear that they give your vehicle an added 3 HP when installed.