I missed those as well. It's an easy fix. I used a pair of calipers to measure the location of those holes in the upper bracket, then marked and drilled them on the bottom bracket. The holes lined up perfectly.
Here's me riveting the brackets in place originally. Nooo Andy!!! Stop!!! You forgot to drill the holes in the lower bracket!
Oh well...

__________________
Andy Compton, PhD EE
RV-10 - #41414 (building)
RV-9A - N643AC (built,flying,sold,missed)
My blood and sweat, the Wifey's tears