Adjuster bolt, likely threads corroded over the years. They always break due to the nut trying to travel on dirty threads. The adjusting flange is also bent, try to get a crescent wrench on it and bend it back. Many folks dont take the time to learn about loosening the 3 main bolts and just start cranking on that poor little tensioner. Get a new one, 2 new nuts and a washer. It looks like a bolt with a washer welded to the end. Classic center item, new one with living in the garage should be good for 50 years!
I just had my 69 280SE torn apart. My steering is very tight, might be looking at a king pin job.