Keystone Hardscapes 995-000160 .5" x 3.75" Keystone Pin. Keystone interlocking pin system maximizes design options, improves the installation process and increases the structural integrity of retaining walls. Allowing for a near vertical or setback installation, Keystone interlocking pins not only save time by taking the question out of proper block placement, they also improve interaction with geosynthetic textiles. Keystone pins are virtually weatherproof, helping the wall to withstand weather challenges.