[STL1944]

I just bought a group of 1947-66 Exhibit cards of Stan Musial (kneeling). In looking at the cards, the bottom border seems to vary across the cards (see photo below). As you can see it runs from no border through a solid black line.

I did a quick Google search and could not find a lot about it. Are these done from different print runs through the years? Any way to tell which one is earlier vs. later? Are the different types collected as different variations?

Thanks!