Thank you to all for your input thus far. I'm currently unable to test it and wondering if I should look for another one, but of course won't know for sure unless it is tested. If my orientation is reversed, the missing pins would be 24, 25, and 26 instead, which are ground, open, and termination power, respectively. However, I checked it again (viewing the connector overhead from the front of the card) and believe that my initial interpretation is correct.

What seems odd is how cleanly finished the metal is where the missing pins are located. There also appears to be a tiny indentation in the center of each of them, and the plastic guard has no evidence that they were removed with a tool. Obviously my wish is that the factory intentionally omitted these pins, but given others' observations and my own sense, I fear not.