This response does NOT specifically answer that question about mishap-rate during 1950's and 1960's. You probably already saw the Report cited below.
Key words might be "mishaps", USAF.
[Some advice from Safety Center, USAF then didn't have any accidents: they had "mishaps".]
Report to Congress, order code RL31571, "Military Aviation Safety, Appendix 2, shows a graph of data since 1950's. A pdf is available on the web.
Rpt states that in 1955, USAF had MORE THAN 2200 Class A Mishaps (contrasted with 70 in 1999). The mishap-rate, shown in Appdx 2, chops the extreme-rate years before 1970, but then-on shows steep drops in rate. [Footnote states that USAF had many more aircraft during that 1950's period, and states that comparisons are not valid.]
For mishaps during an earlier time-interval, found this:
"General George Price, Chief of USA's AAF Office of Flying Safety, revealed that during the war 22,000 Army planes were completely destroyed in non-combat accidents and that 26,000 lives were lost . . . " [Quote from The Air Line Pilot, Dec'45, pg 1, under "Late News" header. ]