The Brothers P were rather late in adding ejectors to their offerings, and King's ejector train is rather complicated and finicky as noted above. When they were first offering ejectors they wouldn't retrofit, but eventually they would. Several firms back in the day offered after-market ejectors for Parker Bros. guns, most notably Moran & Wolfersperger in Chicago and later Gus Habich in Indianapolis.
After Moran & Wolfersperger broke up, Gus Habich took over doing the ejector conversions and Mathew Moran went to work with F.P. Stannard.
I believe the gent on the left has done the ejector conversions on Parker guns --
