It may have been a regional thing. Here in the US I've only ever seen that same one, plus the MS15AV of course, and I've seen one or two Australians posting about the other, squarer one.
The Multiple Scan 15 was basically the budget/cheap display most/all of its life, so I don't think Apple felt any particular need to have them conform to a specific standard, the way a Multiple Scan 20, or later AppleVision/ColorSync displays would have.