The Long Insert is designed and engineered to slide into the long cell cavity of the stretcher block. The ribbed and bottom-notched side should be installed to the exterior face of the block. The bottom notch allows for the horizontal joint mortar to weep to the inside of the block without interfering with the insert installation.
The Short Insert is designed and engineered to slide into the smaller cavity cells of the stretcher block. The ribbed with a middle vertical notch along with the bottom-notched side should be to the interior face of the block. The notches allow for both vertical and horizontal joint mortar to weep to the inside of the block without interfering with the insert installation.