Most dictionaries list only the standard form PROPRIETARY, so it is arguable whether a word list should contain "PROPRIETORY": although Merriam-Webster and Wik

Most dictionaries list only the standard form PROPRIETARY, so it is arguable whether a word list should contain "PROPRIETORY": although Merriam-Webster and Wiktionary (unlike most dictionaries) list it as an alternative spelling, and it does occur a few times in the wild (e.g. see https://books.google.com/ngrams/graph?content=proprietory%2Cproprietary https://books.google.com/ngrams/graph?content=proprietory%2C... for a comparison), it is not surprising that most word lists leave it out. (Or, even if it occurs in a word list, Stephens may have excluded it, and there is sufficient justification for doing so.)