Major Oil Leak! Need to know what bolt this is!
 

1981 300TD. I've got a major oil leak which my mechanic has traced to a bolt which appears to be about 17mm or so, which goes into the front of bhte block, between the crank pulley and vacuum pump.
Apparently there is some kind of speed sensor mounted on a bracket in front (over top) of this bolt.
Well, the bolt has backed out, is loose, and I'm losing a ton of oil.
My mechanic was able to tighten it using a screwdriver, of all things, but states that he doesn't think he can get at it with any kind of driver.
What is it?
How to get at it?

here's a pic: