These are replacement airflow caps for the Quartz Quest NE by Divine Tribe. Like the originals they have the airflow-in tube built in to the cap so these do not need to be replaced separately.
There is a little variation in these caps, you may find you have one that fits but does not turn as easily as you'd like or may not quite fit the Silicone 510 water tool adapter. I (Matt) replaced the one on my test unit as the adapter wouldn't quite fit, though looking through them it seems I was particularly unlucky with the one I chose. The rest seem to have more consistent quality caps so you are likely to avoid this situation. If you are buying a replacement because the adapter does not fit your original, leave an order note at checkout and we'll check the replacement for compatibility before packing it!