Brazed, is like soldered but at a higher temp using a brass alloy as the bonding material.

if the vents are brazed, they can be removed with heat, it just takes more than if it was soldered. After they are removed the threads can be cleaned with a tap and you can then thread in another vent.

If they are not threaded in then re-brazing will hold them in.

It would seem that Knight wants to sell replacement breech plugs and not replacement vent liners.