Any surplus product may be sold in the open market under such terms and conditions that it shall not compete unfairly with the product of free labor. All goods, wares and merchandise, made by convict labor in any such institutions, shall be branded, labeled or marked as required by chapter 31 of the title Property (s. 46:31-1 et seq.). Any other nonperishable articles so marketed shall be stamped in a legible and conspicuous manner “Manufactured in the New Jersey ,” naming the institution of manufacture; provided, that whenever from the nature of the articles made or manufactured it shall be impracticable to stamp each article, or if such articles are usually put up in packages, it shall be sufficient for the purpose of this section to put a stamp, label or tag upon such package, showing where such articles are made, provided that said stamp, label or tag shall be put on said package in a good, lasting and permanent manner.