so the symbol is at least from 1968 if not earlier! Dreyfuss does attribute some things in the book to himself but he doesn't in the Material Handling section so it's not clear if he was involved in the production of the ANSI symbols or not. in 1971 he did some different work for ANSI which means that me googling "henry dreyfuss ansi" isn't gonna get me super far on this question probably!
i haven't found any other references online to the 1968 version of ANSI MH6.1 but carnegie mellon apparently has a copy of the 1972 in their library and hey what if i went to pittsburgh? that's like only an eight hour drive from here