At the Varnum Memorial Armory Museum, we now have a rare 39-star American flag, which was in use for … zero days. Period printed parade flags are occasionally encountered, but generally are uncommon. Flags of pieced and sewn construction in this count (39) are extremely rare with likely less than ten known examples. 39 stars is an unofficial count, because of the introduction of North Dakota (39) and South Dakota (40) at one time, instead of as a single state (formerly the Dakota territory).
Despite the fact that the U.S. went from 38 stars to 40 stars in a single event, 39-star flags are actually more common than 40-star flags, which are extremely rare. Since people anticipated a single state, rather than two, manufacturers produced reasonably large numbers of new flags of the 39 star count, only to find that they would never be official.