Differential Cover Low Plug Or High Plug

So I picked up a Dana 30 differential cover for my damaged 27 cover. This one that I got has a higher fill plug then the one on my 30 axle but same as my 27 installed in my 71 now. It can't be too big of difference, but I wonder which is more appropriate.

 

I'm confused, is this going on a 27 or a 30?

 

I was told that the 30 could be used on a 27. I was going to install on a 27. I now see 30's with either a high or low plug. I have two 27's and two 30's seems each has a high and a low.

 

So if I use the high port on a Dana 30 I should fill to one inch below the hole I guess.
Correct me if I am wrong.

 

I tend to run mine a little over full I like knowing everything is getting oiled.a higher plug would be nice.

 

How low is the low plug? Some covers have drain plugs. My D30 cover has one (aftermarket), I love that feature.