Some are. What I think will fit: mainsprings, mainspring assembly (cocking rods and detainers), the underbolt and the safety assembly (this, as I was told, can even retro-fit to pre-1974 "M" actions, which will thereby be converted to auto safety). Probably also hammers, sears with their springs and triggers, (provided Izh-43 is not a magnum action, which has independent firing pins). Some fitting to place can be expected. All the above for DTNE guns. Single triggers of course not, ejectors definitely not, and the ejector cockers on the action are also different. Izh-43 stock can be filed to fit Izh-58MA, but not vice versa.