The spacer is made for a 14.5” carbine barrel with a standard flash hider to make it 16” legal, instead of using an extended flash hider.
If you have a 14.5” carbine barrel then use an extended A2 flash hider from BCM or Luthe-AR and it should work fine with the bayonet and be pinned 16” legal.
A 14.5” / 14.7” mid-length is another animal altogether.
ETA - There may be other extended flash hiders or brakes that will work not only with bayonets but also with suppressors, just depends on what you want or need.