A patent disagreement arose in 1938 with Lincoln(another manufacturer.) So it could predate that date. 20's or 30's suggested per web discussion anyway.
Grease Guns (CJ3B Bulletin Board)
https://www.google.com/search?q=ste...=X&ei=5_nTVLrlI8GLNpuqgsAC&ved=0CB0QsAQ&dpr=1