These Griffin Plan A mounts are intended to make your silencer compatible with Griffin TM style muzzle devices.  The following is a quick reference for which mount fits which silencer:

• 1.375x24 - This is becoming a more and more universal mount thread pitch for many rifle silencers.  Currently, this Plan-A will fit things like the Dead Air Nomad, Silencer Bravo style silencers (omega, hybrid, harvester big bore, etc), YHM Resonator R2, etc.
• 1.125x28 - This thread pitch has long been used in the pistol silencer world.  SilencerCo Octanes, Rugged Obsidians, Griffin Revo/Resistance, etc, all run this thread pitch.
• Charlie - This thread pitch is compatible with SilencerCo Saker and Chimera silencers.