The bottom, which is shorter is the correct bolt. The longer one is for 70 model year and the NOS later shackle kits come with that long bolt also.
I agree...i have the originals on my bench and the bottom pic and nut are the originals.
'69 RS Z/28 Olympic Gold, 711
Flat hood no spoiler, black top, endura, 4:10
69 X77 Z/28 69 711 Original Paint Unrestored
'69 X66 Convertible Cortez Silver 712 black top Endura, auto, bumper guards, am/fm rear speaker 44k miles