Nope, I can absolutely unequivocally guarantee is was 21 from the implementation of NSNs until approx 2002/2003. Then we went to 20 cause 22 was already taken.
Off the top of my head, the Kent Moore double flaring tool was a 5120-21-111-3075 stock number. Very early number, with a 21 country identifier.
Many small arms (like the FNs and Enfields)and their parts are in the 1005-21-103 range as well, again stuff from the 50s.
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