This was a modification of PIR-2 with elements of PIR-19. Distinguishing feature is that the fuze head and rocket motor adapter are made in one piece. About 0.33” longer than PIR-2 fuzes, apparent in extension of magazine. During March-June 1945 about 50,000 were manufactured and loaded by Caltech.
There is no information on how this fuze differs from PIR-22. PIR-22 was made for the 5.0” HVAR and was 6.02” long. It was built to be more impact-resistant and more reliable than PIR-20 (Mk164). Preliminary tests were done in September, 1945.