Can you tell if that is all I'm missing from this pic? This is how it arrived from eBay.
You are missing the Rod Clamp,.. washers..wingnut.
The Rod Clamp/wingnut isn't sold separately as there are too many styles..and there are different diameters as mentioned above 6mm or 8mm.
I have no idea what came with the Pro-X or the underneath setup.The DMPad Hi Hat 12 (attached) had a slot but was also rounded at the top (Cymbal Pivot) where as the STRIKE Hi Hat is flat so the anti-rotation nut may be different..or exactly the same.
The one for the STRIKE Cymbal has a bottom nut with a tang/tab for locking the cymbal in place..anti-rotation...the
STRIKE Hi Hat clutch is going for $100 which is extreme (obscene)..
There are cheaper alternatives that you may be able to scavenge parts off of , however it's usually not mentioned what diameter wingnut is used etc..so it's a guessing game unless you know the hole diameter of what you have and the wingnut size of what the other clutch is.
I have 5 STRIKE HH Clutches..3 are 6mm..2 are 8mm..so there may have been a design with the earlier 12" w/bottom and the later 14" with boot
or supplier change for the whole Clutch setup.
Hi Hat clutches are typically for Acoustic Hi Hats which are thin pieces of metal,which is why there are felts so some of them may be too short for an E-Hat Cymbal
You can go through ebay listings/pics and see if any are similar and matches up to what you have....bottom line.. cheaper/simpler to find a complete substitute for the clutch instead of missing pieces..unknown if the thread pitch of the hollow threaded is the same as the Alesis anti-rotation nut.
GenericSPLGIBRALTARALL Hi Hat ClutchEDIT:Added Links,Image