I'm no expert at these things, but my 2c worth:
The newer 586/686 retaining screw has a spring loaded pin on it, so if you are shooting "warm" loads, the screw probably doesn't have to get that loose to let the yoke bump past the pin...

I'll go even further out on a limb and say that the spring loaded pin is part of the "dash 3 new yoke retention system".