Notice the rounded threads ( approx 4 threads) . This is what makes this cap easy to identify once you have lost it. This cap part number at http://www.waterwayparts.com is 602-5321. It will also require an o-ring (805-0224)
Below are images of the same model sand filter tank drain (pre 2011) that requires a different drain cap. Notice the threads are not as rounded and there are approx. 8 threads. This cap part number at http://www.waterwayparts.com is 602-5301.This cap also requires the same o-ring (805-0224). 
Archive | July, 2015
waterwayparts 