BUFF,
As usual there was a transition, so the best we're going to be able to pin it down is to a period.
From my personal observation, the earliest domed screw is on an N frame #S96911 from 1953. The latest flat screw is #S146567 from 1956. Everything after that has domed screws.
Therefore it's at least a 3 year transition period. This appears to be across all frame sizes. Maybe some other posters can help us widen that period which I expect it will be and include a few far out.