Probable Source of Probability-Impact Matrix Concept

MIL-STD-882 SYSTEM SAFETY PROGRAM REQUIREMENTS Military Specifications and Standards was first published in 1969. Its June 1977 version introduced the "risk matrix." I'm not sure if this is the source of the probability-impact matrix concept, but it is certainly a strong contributor to its popularity.

See safetycenter.navy.mil/instructions/osh/milstd882d.pdf