hopefully last thing HELP with adding rear DIFF oil

[hjalbert]

on my rear diffeential, i have this plug under the half way point, then on top i have this 3/8 inche fitting, that opens up into the rear diff, nothing covering it, it was packed up with grease. i am thinking that is where i add fluid, but havent seen anything like that before, was wondering if anyone else had, it is on the top to the passengers side, pointing up. its about 1/2 inch long, was guessiiing one of the previous owners broke the plug or something and put that in to make it easier to add fluid. thanks harry