Ask Trade Member Dale. He literally wrote the book on Hawaiian shirts : ).
Here's a fairly reliable formula for using RN# to determine date (although if an early WPN#, used 1949-51, numbered 00101-> 13669, was swapped for an RN#, that complicates matters):
First RN# was 13670, issued 1959
~ 2635 numbers have been issued per year, whether used or not
THUS:
x (your RN#) - 13670 =
y / 2635 =
# of years + 1959 ---> earliest your garment can be
Checking against known manufacturing dates has shown this formula to be fairly reliable.