Different manufacturers like their products to not quite fit anybody elses. Having a keyed case and a matching shaped plug stops you plugging a servo in backwards, but not if you stray from their true path.
Fortunately, with modern plugs with 3 connectors, plugging them in reversed does nothing since the ground line from the RX is connected to a dead end, and the signal pin connects to the servo ground line, the pos power staying put.
I have always found that the safest bet is to figure out where the RX board is, and assume that the pins nearest the edge of the board are the neg, or ground, line, and plug the servo in with the black or brown on that pin. On some sets, it is quite easy to shove the plug between the rows of pins. I have found that this doesn't work, either.