Working on my Landcruiser. The bolt in question is entitled Lock bolt in the post linked below.

http://forum.ih8mud.com/80-series-tech/185209-changing-your-belts-25-minutes-3.html#post4244791

It's the bolt that locks the adjustment bolt in place on the alternator. It's a 12mm external hex head with a star inner head. The external head rounded (or was rounded already) when I tried to loosen it to replace the v belts today. Alternator won't move if this bolt is tight. I got a star bit inside it but I'm afraid the star is about to strip too. I've got a fair amount of torque on a 18" breaker bar and it's not budging. Problem is complicated by the fact that while the picture above is with the radiator removed, my radiator is in place and there have to be enough extensions to get the breaker bar beyond the belts. This causes whatever socket I am using to twist off line pretty easily. I think that's probably how it rounded to begin with.
I bought a set of extractor sockets from Craftsman and using one of those is the next step. The bolt threads about 3/4" into the alternator itself and I'm guessing the dissimilar metals of the bolt and the alternator housing has caused it to seize up.

One option is to remove the whole adjustment bracket from the block, disconnect the alternator and remove the alternator with the adjustment mechanism attached. This may not be feasible since there is very little room to remove the alternator to begin with. I haven't yet figured out what space the alternator itself would fit through to get between the fender/chassis and the engine.

All solutions considered.

__________________
1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13