The bladder has a female thread for a bolt.
You will find that what you have been supplied is a length of 1/2" UNC threaded rod that is screwed into the bladder. It extends beyond the lower surface of the piston so that it can be used to secure the assembly to your mount. You can shorten the threaded rod if you wish and use a nut to secure the bladder to the piston. Then use another method to secure the assembly to your mount - remember that on full compression the bladder will roll down the piston and beyond the outer lip at the base.
BTW, the piston diameter increase at its base is to increase the spring rate when the bladder is on that part of the piston.
Edit:
You can see the threaded rod in the pic I posted earlier. If you cut the threaded rod the nut will go up at the top of the recess (or you could use a bolt. Only friction between the bladder and piston are holding them together without a bolt or nut and threaded rod. You can separate them but be warned, take care not to extend the bladder as you will have trouble getting it back to the working position on your vehicle.

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