I just got out my copy of Dreyfuss's 1972 "Symbol Sourcebook" which I thought I had checked months ago and it turns out it's in there! "Symbol Sourcebook" is a big book of symbols and for each section Dreyfuss has a bibliography and for the Material Handling section, there are several potential sources but the most likely looking to me is either the ANSI "Pictorial Markings for Handling of Goods" or its ISO counterpart
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!