Code of Federal Regulations Title 31. Money and Finance–Treasury § 31.91.7 Gambling

(a) Participating in games for money or other property, the operation of gambling devices, the conduct of a lottery or pool, the selling or purchasing of numbers tickets, or any other gambling in or on the property, is prohibited.

(b) Possession in or on the property of any numbers slip or ticket, record, notation, receipt, or other writing of a type ordinarily used in any illegal form of gambling such as a tip sheet or dream book, unless explained to the satisfaction of the head of the bureau or his delegate, shall be prima facie evidence that there is participation in an illegal form of gambling in or on such property.