Name That Part - FX Wildcat

Can someone provide the technical term (or part number) for the doohickey I'm pointing at in the attached photo? It sits directly above the junction of where the air cylinder meets the breech.

Need the nomenclature to support an email in which I need to describe this part's failure to hold back the violence captured inside the Wildcat.

Thanks.

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"sniperlabyo"my heavy duty HP valve also fit the WC. stock bore is 5.5mm ID. mine can be bored to 5.0, 5.75, 6.15mm ID, they don't break on the WC, but if you want higher speed than stock go for a bigger bore if you want 33g in the 865 range. 

Do you put your value in the WC when you tune one. If so I will not need one, you did a tune job in Dec 2016 for me. If not I'LL email you for one.