I had to pull out one of my stock retainers to see how the locking part of the retainer corresponded to that flat spot you were talking about.
That flat spot needs to be back toward the rocker. I beleive that is a relief to allow additional clearence for the rocker body. The flat spot on the retainer is exactly opposite of the wide opening where the valve comes through the retainer & locks onto the valve.
So Flat spot on outside of retainer towards rocker. Opening that allows the valve to even come through the retainer away from the rockers? Hopefully that will clarify things a bit.