GM moved to use the round key in the door locks and started coding the doors with the trunk locks in around 1971 if I recall correctly. They also stopped putting key codes on the lock case at this time as thieves learned that pulling a door lock would net them with the key code for the ignition as well. It was said that GM instructed B&S to change this in order to assist in cutting down on car thefts from keys being cut to code. I have forgotten so much of this stuff over the years that I opened a few of my old Briggs & Stratton technical manuals tonight and refreshed myself on a few service bulletins. The dust shield door itself is pushed open by the key and that gated slot in the shutter face used to fit the side milling of the key blade. Upon being fully inserted the key would park the dust shield at 90 degrees and it fits into an opening that was cast into the core of the lock. When GM and Briggs shortened the keys up in the late '70s, they also shortened the side milled groove in the keys and the dust shield had to be changed to eliminate the gate as it didn't work well with the shorter keys including most of the aftermarket blanks from Ilco and Curtis which would bind.
So it's now been about 45 years since they redesigned the dust shield shutter. If you use the original gated shutters with the newer or shortened key blanks, chances are it will bind and/or break inside the lock when you put the wrong blank into the lock. With that being said, if you're going to use the old original shutters, make sure you stick to using the older key blanks and not the newer ones with a shortened slot milled onto the side of the key blade. These old original keys are becoming harder to find nowadays so if you see some NOS GM key blanks, buy some and have them cut to code by your local locksmith. It's better to have them code cut rather than duplicated since code cutting is much more accurate on both the spacing and depths. Any good locksmith can code cut them just a fast and any duplicate key is cut. If you have your key codes it makes it even easier.
Your car will love you for it.
