No person can be convicted of any offense under the last two sections¹ by reason that the contents of any barrel, box, case, cask or other vessel or package mentioned in the bill of lading, receipt or other voucher, did not correspond with the description given in such instrument of the merchandise received, if such description corresponded substantially with the marks, labels or brands upon the outside of such vessel or package, unless it appears that the accused knew that such marks, labels or brands were untrue.