Fox Fit CTD Bleed Fitting. 
 
This is a WSS brand, model STBP1108 bleed fitting, which is equivalent functionally to a Fox 803-00-809. 
 
This bleed fitting threads onto the MTB Hydraulics bleed syringe like other Fox bleed fittings. 
 
This fitting is used to bleed Fox CTD dampers through the top damper bleed port.