It's only a marketing tactic. The EVM only came in 2 power handling versions spread across 3 different baskets, 200w and 300w, those are conservatively rated. Series I, Series II and the Pro-Line all were used by Mesa at different times. The speakers could handle much more wattage than their conservative ratings. Because EV was in questionable financial shape in the 80's they would put whatever rating an OEM buyer would want on the sticker, as in the case of the Black Shadow. Again, these speakers can handle well in excess of their ratings. The weak link would be the Series II because of production problems at the time.
They're all the same speaker