I believe the magnum twenty was introduced in 1967 and was only built with a "round knob" or prince of whales style stock for 67 and 68 in 69 it was produced with the traditional pistol grip stock you have, the barrel being a jap barrel hurts the value considerably, also hurts value if it comes with the plain barrel. The crack also doesnt help, i wouldnt pay much over 550 600 max with the description you have given us and pictures, if it was all original with belgium vent rib barrel no cracks would be around the 1000 range give or take. I have the identical gun from 69 mine is original with no cracks and it came with a mod vent rib and a ic plain barrel and i paid 950 tax in, just to give you an idea