That's called an eye bolt.  The part number is 027 260 937.

It's present on my '85 and it does make adjusting belt tension a breeze.
  It's missing on my '84, however, and I secure the compressor with a
standard nut and bolt through the front of the bracket on that one.

Mike

Jean-Woo Kim wrote:
> As part of a head gasket replacement job, I was removing the A/C compressor
> on my 87 Westy with 2.1 WBX and found that the tensioning bolt had been
> sitting alone. The receiving nut/bracket mounting is missing entirely. I
> imagine the part was shaped like a cylinder with threaded hole in the
> middle. I don't know how it kept tension on the A/C belt because A/C was
> working fine. Does anyone have this part? I am willing to pay for it. Or
> could someone take a picture of it so that I can try to fabricate one?
>
> J-W
>