Hi guys, I need some help.....

I have a 9 inch rear end on my 1958 ford fairlane. I want to top up the gear oil in the rear diff but I can't find the fill hole anywhere. I can see the drain plug but no fill hole.

On the drivers side there is a flat on the side of the housing and this looks like where the fill hole should be.

I realize there must be a fill hole somewhere but I just can't find it.

Any help much appreciated, thanks, James