The article further down this website page, titled The Day The Corps Changed....
seems to indicate that McKeon eventually retired USMC in 1972; however, I do not believe that to be correct, as I and McKeon checked out of Cherry Point Disbursing at the same time in 1959--I was going to HQMC, and McKeon was going out on a medical discharge.
I was a S/Sgt at that time, and to the best of my recollection, McKeon was a L/Cpl--however, others have argued w/me that McKeon was a Cpl (E-3) at that time having been promoted prior to the new (crossed rifles) rank structure prior to 1Jan59--they may well be correct on that point--it has been a long time and memories dim on the finer points.
I do recall discussing the USMCR w/McKeon at that time and I had myself previously taken my discharge and joined a USMCR unit in Providence; McK seemed receptive to the idea as there was a USMCR unit in Worcester, Mass. So he may well have joined, and retired, USMCR--but I personally doubt that he ever returned to active duty, else that would have pretty much been common knowledge in the Corps; nor does John Stevens mention anything relative to this in his book--Court-martial At Parris Island...
Hope this helps.
Anyone viewing this who has factual information on this, please post information/sources--Thank you.