I have that same indent too on my 67 brackets. I'll look for that bolt style with the captive star washer tomorrow in my bolt collection. If I have it, it's yours free.
As a side note, my bolt head isn't indented like John's 69. It has a flat surface with 3 lines and 'RS' and is dark phosphate. I'm sure they must have used different styles. I'll check tomorrow morning.