I think I'm missing a nut.

I would definitely add one, some of the old Chebbie 265s didn't have balancer bolts. A friend had one and I road to school with him from time to time in the 60s. We crossed a set of train tracks and he had it wound up tight. To make a long story short, the balancer hit the pavement, two pieces came through the hood and one of those came though the windshield. Luckily we weren't hurt, but we had to make a trip back home to change our shorts.