The place I bought my .308 carried the entire Saiga line, including the gun in the picture. When I asked them (at the store and the gunshow) about the thumbhole I was given vague answers that basically amounted to "No options for your gun". I find it funny that there is a .308 model still advertised showing this stock, and one of the vague answers was "well.. maybe it can't handle the stresses of a .308".
I have a feeling that if I had bought a converted gun the answer would be different.
Then again, I've gotten vague answers about the 20rd mags as well.