As I pointed out int that thread I don't think it means anything. The first use of Junior was on the 1906 model which was the first one to use the new rib back blades instead of the wedge blade, they continued to use it on later models to designate they used the new style blades. Why they continued using it on some later when the wedge blades were obsolete is beyond me. Slivovitz pointed out the difference in the tabs, the Junior is smooth, the non-Junior is indented but I've since found out that's not accurate. Here's a picture of three of mine , the Junior on the left has a smooth tab, the Junior on the right is indented, just like the non-Junior in the center. As you can see the center one is identical to the one on the right other than the word Junior.
It obviously served a purpose on the early ones, but I doubt we'll ever know why ASR stuck the Junior name on some of the later ones and not others.
One has the word "junior" stamped on it. That is the only difference. I think they just started calling the 1912s juniors after GEM starting coming out with a lot of different razors and they needed something to call the old 1912.