Tokyo Marui MK18 nozzle assembly help

[Kayos]

Hi all, I ordered the DP spring set for my TM MK18 GBBR, upon disassembling I noticed that there was no firing pin o ring from the factory.. the spring set came with an o ring however it does not stay on the neck of the firing pin .. does anyone know whether the mk18's don't require a firing pin o ring? Seems weird as all MWS's I've seen have one ... any help appreciated.