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Flag of the United States: Flag Code
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The United States Flag Code outlines certain guidelines for the use, display, and disposal of the flag. For example, the flag should never be dipped to any person or thing, unless it is the ensign responding to a salute from a ship of a foreign nation. (This tradition comes from the 1908 Summer Olympics in London, where countries were asked to dip their flag to King Edward VII: the American team captain Martin Sheridan refused, famously proclaiming that "this flag dips to no earthly king."[12]). The flag should never be allowed to touch the ground and, if flown at night, must be illuminated. If the edges become tattered through wear, the flag should be repaired or replaced.
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Prior to Flag Day, June 14, 1923, the federal or state had no regulations governing the display of the United States' Flag. On June 14, 1923 the National Flag Code (U.S Flag Code) was constructed by representatives of the Army, Navy, and other groups.
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Although the Flag Code is U.S. Federal law, there is no penalty for failure to comply with the Flag Code and it is not widely enforced—indeed, punitive enforcement would conflict with the First Amendment right to freedom of speech. Passage of the proposed Flag Desecration Amendment would overrule legal precedent that has been established in this area.
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This code includes the provisions of the code adopted by the National Flag Conference at Washington on June 14, 1923, with certain amendments and additions. Since it is the patriotic duty of all to display and use the Flag appropriately, the provisions of the new code are supplied for your information and guidance.
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This work is in the [P]ublic domain in the United States because it is a work of the United States Federal Government under the terms of Title 17, Chapter 1, Section 105 of the US Code. See Copyright.
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The answer seems to be that only 50-star flags are "official" but it is appropriate to display earlier examples. A publication sponsored by the Boy Scouts of America states "Historic U.S. flags are due the same honor and respect that are given today's colors. When a historic flag is carried or displayed with a present-day flag, the modern flag takes precedence." These do not appear in the Flag Code nor the Executive Orders covering the flag, but they make sense.
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Flag Code