In what circumstances will a federal court overturn a conviction for possession of marijuana after illegal importation?

In September 1968, appellant was indicted for concealment and sale of marijuana after illegal importation, 21 U.S.C. 176a and 26 U.S.C. 4742(a). After trial to the court and conviction, he appealed and this court in United States v. Cepelis, 426 F.2d 134 (9th Cir. 1970) remanded for further proceedings. We asked the trial court to make clear by special findings whether it relied on the statutory presumption of illegal importation for one in possession of marijuana or upon admissions or other evidence.