The 108 and 110 have OCC solid center pins treated with their alpha process.
The 106 has what they call their "filament" center pin (which is comprised of small spring tensioned wires) and those are phosphor bronze, probably because copper would not stand up to the repeated tensioning and release of connecting and unconnecting the rca plug. This is why pure copper is not usually used in electrical outlets, etc.
The 106 does have set screw connections (same as the 108 but not the 110), so no soldering, and it is a fine sounding connector. The 106R is used in their Reference line of finished cables while the 110G is used in the Evolution line. Interestingly, for their flagship Lineflux cables, they use the CF-102R rca plugs that have a solid center OCC rhodium plated conductor, like the 110 plugs used on the Evolution line.
All their plugs work and sound great, and you can choose between the typical sonic differences between rhodium and gold plating. I personally like the solid center pin over the filament pin, just personal preference.