The bolts go in on the side where the drive shaft connects. There are 6 washers, and the washers should be against the flex disc. So on three sides the washers will be on the bolt head side, the other three it will be where the nut it.

The raised boss on the flex disc seats into the flange, so the flat part does not contact a flange.

Some more info here:
Flex Disc Orientation, safety and durability issue.

He said he used the allen bolts that came with the flex disc, so I don't think this happened because he re-used the old bolts...

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"The MB W123 is so bulletproof, you can drive them forever. Which is a good thing as it takes that long to get anywhere."
Betsie: 1984 W123 300D (hobby, 280k miles)
Myrla: 2001 Mazda Protege 2.0 ES 5spd (daily driver, 130k miles)
The Turd: 2007 Toyota Camry (wife's car, 118k miles)