1958 9 inch rear end fill hole

[1959retractable]

I agree with Ford Only. It is likely someone installed a differential gear assembly that has no fill hole and that they installed it in your differential housing with no fill hole. I have seen this before and it's annoying. There are kits where you can drill and tap your own plug but you have to guess where to put it and then deal with shavings in the rear end. I know one guy who removed an axle and poured gear oil in there. Unfortunately, most people end up having to get a differential gear assembly with a drain plug in it and install that in their differential housing. Sorry to be the bearer of bad news. If you Google Ford 9" Fill Plug Kit you will find some weld-in kits.

I have seen fill plugs use a square drive from a ratchet (both 3/8" and 1/2"), allen wrench plugs and hex head plugs requiring a socket. Many differentials were switched out over the years. My '59 has an allen wrench fill plug. Not sure if that's original or not but I prefer the 1/2" ratchet plug.