Hesse, the springs are different. They are almost a reversed version of eachother and the prongs on the D spring are open when the spring is relaxed, while the F prongs are closed over. The D spring is 604260, and the F is 609229

Sounds like some of that good old new engineering to same dime they have adapted to weedeaters with the tab slot in the molded plastic housing that slips out and unwinds the spring. Found that out when I brought 25CC Craftsman weedeater off ebay Due to the cost of labor if was cheaper to just wholesale the new return units than fix them. But, at least I did get at 15 bucks full serviceman's discount. My labor was two hours, in my case I anchored the spring tab with little screw and washer also.

Are you saying that the catches do not extend with the centrifugal action of pulling the starter rope, or that nthey do not extend enough?

I agree with Tomplin, sounds like when the old cord broke the recoil uncoiled with enough force to snap the hook end off, or the force was strong enough to bend the hook end and now the spring is no longer able to wind. Make sure that the cord diameter is the same as what Toro specifies, if it's too thick it could take up too much space inside the reel and bind when it's retracting.