If you already have a SIG PCB folding brace but you are tired of the rotating arm cuff, you can easily convert it with this adapter from Farrowtech. This adapter converts the SIG brace end to the A3 Tactical Steady-brace. Simple 10-minute installation. Made from SLS Polymer.
For calculating length of pull, the overall length of the SIG brace in this configuration is 9.25" from the rearward face of the Picatinny rail to the rearmost part of the Steady-brace.
13.4375" Length-of-Pull on Sig MPX/MCX