The stem you have is a Savoy, there are a number of different similar stems. I don't know which yours is and they don't all use the same size special wrench. Mat manufacturing (won't sell to you directly) makes two different wrenches that fit three different stems - part#s 1890 & 1891. Kissler's (Quality Repair Parts) # 08-9415 fits Savoys with 7/8" & 3/4" flats. There are no doubt others.