Ohlins TTX22 and STX22 Bleed Fitting. 
 
This is a WSS brand, model STBP1120 bleed fitting. It is effectively a long reach M4 bleed fitting with a conical seal interface. 
 
This bleed fitting threads onto the MTB Hydraulics bleed syringe like a Fox bleed fitting. 
 
This fitting is used to bleed Ohlins TTX22 and STX22 dampers through the damper bleed port. 
 
IMPORTANT NOTE: SOME TTX22 SHOCKS REQUIRE AN M5 BLEED FITTING. MAKE SURE TO CHECK WHAT THE SHOCK IS BEFOREHAND.