M&k ss-150 thx tripole

hi all, i was just wondering if anyone could tell me why some of these speakers have the directional arrow sticker on the back and some dont?

for the ones that dont, i dont think the stickers have fallen off.
i know that there are some that say mk II and some that dont, which means they are a bit different, however, im not sure if maybe the direction did not matter on some of the earlier models. it seems that alot of the mkII all have the directional arrow sticker on the back.

thanks for your help
In a dipole, electrical phase is in sync while in bipole it's out of sync. That out of sync condition places the positive cycle of one side ahead of the other and thus the desire to directionalize markings on a "matched" pair so that the left facing of one tripole and the right facing of the other are in sync, placing the positive cycle of both towards the front (or rear) of the room in a side surround application or between (or outside) in a rear surround application.