You were right - 7/16" 20 pitch. Bought 2 at Home Depot and they fit perfect (I bought Grade 5 as I didn't see any Grade 8 at HD).
Trying to figure out how to secure the shoulder belt bolt here and still allow movement at the connection point. I believe seat belt bolts have either a spacer or have some kind of shoulder - so you can tighten the bolt down but still allow the belt to move some.

ryan

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RG Newell

1984 300D
1972 250
1986 560SL
1991 300CE