Thanks to the generosity of Chuck Melton I do now carry a 17mm plug in my toolbox.
Years ago my neighbor helped me unplug the ZF by getting a 11/16 bolt from his supply, welded a nut to it, and ground the head slightly to the 17mm size. I later added a 17mm bolt with a double nut configuration to make a total of 3 tools for my boxes. One remains home, two travel with the Pantera where one bolt setup is ready as a “give-away” as needed.
So, for those with metric inventory, look for a 17mm bolt. American types, hunt down a 11/16 bolt and then file the head to fit.
I prefer Chuck’s socket but both alternatives work fine during the many TPOC tech sessions held here.