19 CFR §10.43

1. (a)The Center director may, at his discretion, require appropriate proof of duty-free status for articles for institutions claimed to be exempt from duty under subheadings 9810.00.05, 9810.00.15, 9810.00.25, 9810.00.30, 9810.00.40, 9810.00.45, 9810.00.50, 9810.00.55, Harmonized Tariff Schedule of the United States (19 U.S.C. 1202).
2. (b)Appropriate proof may be a copy of the charter or other evidence of the character of the institution for the use of which the articles are imported.