Bobbins are a component of all Halkey Roberts type automatic inflation mechanisms commonly found in auto-inflating PFDs. (Another device sometimes used is an actual pill-like tablet.)
They dissolve when immersed in water, allowing a spring-loaded puncture mechanism (which they have been holding back) to pierce the CO inflation cylinder and inflate the PFD.
All Mustang automatic inflatable PFDs come with cylinders and bobbins.

This bobbin is used in the following PFDs
MC1900
MC1900 HR
MD3017
MD3019
MD0200
MD3053
MD3054

We recommend annual maintenance and inspection of your PFD.
For recreational use, bobbins should be replaced every three years to avoid inadvertent inflation. Replacement is required, of course, anytime the PFD inflates during use.

Always refer to the label inside your inflatable PFD to determine or confirm the type of mechanism used in your particular inflatable PFD.