Hard y rod, The bolt that holds the brake strut to the swing arm is as WacoBrian says, A shoulder bolt.
The one that is in the backing plate end has a head that looks like it was round then had the sides ground down so they are flat. The one on the swing arm end Has a regular head.
I'm not sure where you can get one, If I had an extra I'd send it too you. Maybe some one on here has one or knows where to get one.
If you have the Clymers book, on page 136 is a pic of the rear wheel and brake parts. Item # 36 is the one you are looking for.
You can try a regular bolt with a nylock nut. It might work. I woulkd use a hard one, grade 8. Hate to have it break.
Leo