You might buy it as a nice collector's item. The practical value is very low. The first generation of the focusing adapter will tend to bind during focusing due to the load of the heavy lens head sitting way in front of the adapter. The only working adapter is the AU-1 which, unfortunately, is much less common.
I'm not talking about the ED version of the 800mm f/8
lens head. This is a very elusive item probable made in <100 items and it delivers amazing image quality too.

This image shows the ED version with the AU-1. Do note the golden ring signalling ED. The entire package is *very long* and *very heavy* and you really need a tripod to match.