That's the critter -- factory bolt is an allen, mine was replace with a standard hex head -- previous mechanic didn't own a long allen driver, I suppose.
Soak the base down with good penetrating oil, attempt to get it under the adjustment plate. Run to operating temp, soak down again, and then let sit overnight. May take several attempts, and as a last resort, you can always find a slide hammer and attach to the distributor and pound it out by brute force -- probably ruin the distibutor that way. There is a 0-ring down at the base that gets seized.
Mine just pulled right out, fortunately.
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!