There are two versions of the mid driver, the "normal" one in the 40 and the pro 25 and pro 45. Then there is the "S" version (for super dome), which is in everything else (50 on up). I remember when I was first getting to know these parts and found out the recone kits for repairing both were exactly the same. The only difference between the two was a larger magnet (larger motor) for greater power - which relates to higher power handling and greater dynamic range. So from a technical point of view there is only one ATC mid dome, with a version that employs a larger magnet. Everything else about it - the coil, the gap, the frequency response etc- is identical. The two versions sound exactly the same, the "S" version in the 50 on up is just capable of more output, more SPL, which matters in some situations but not all.
Brad