Construction sheet for the 1912 (48-star) <a href="https://en.wikipedia.org/wiki/Flag_of_the_United_States" class="extiw" title="w:Flag of the United States">Flag of the United States</a>.  This is presumably the specification from the U.S. Navy published in conjunction with <a href="https://en.wikisource.org/wiki/Executive_Order_1637" class="extiw" title="s:Executive Order 1637">s:Executive Order 1637</a>, which defined the flag.
U.S. Department of the Navy
1912
From page 19 of the archive.org version of <a rel="nofollow" class="external text" href="https://archive.org/details/starsstripesfrom00stewiala"><i>The Stars and Stripes</i></a>, 1914, by Charles West Stewart.  This is printed there alongside Executive Order 1637, which defined the flag, so presumably this is the specification from the Navy Department mentioned in the order.