What is the o-ring, English – Sony MPK-THB User Manual

MPK-THHB, MPK-WF, MPK-P5, MPK-PEA, MPK-THF, MPK-WB, MPK-PHA, MPK-THC, MPK-TR3, MPK-THGB, MPK-DVF7, MPK-PHB, MPK-THE, MPK-P9, MPK-NA, MPK-THG, MPK-WA, MPK-THD, MPK-WE, MPK-WD, MPK-THH, MPK-WEB, MPK-THA, MPK-THJ