If the last quarter turn of your Gillette adjustable does not works, maybe you are thinking that your machine is broken, or that your retailer has cheated you. It is not true. This mechanism uses to fail usually, but it is not broken.
One question that is usually asked on shaving forums is that if the last quarter turn must work always, on every adjustable Gillette machine.
The answer is clear: yes, always.
So, is my machine broken? Probably not. Maybe it is just not lubricated enough.
You only have to make some checks. It is not necessary to disassemble your machine. Even it is not recommended to do if you are not an expert, since it is possible that afterwards you an not adjust the head and the blade gap correctly.
When you turn the tip of the handle to close the doors you will notice that when the doors are closed you can make a last quarter turn that seems to be a little harder. "A little harder" but not much harder. It does not have to make "squeak" sound, or strong scratch. Just a smooth friction.
At this point you notice that the doors (which are closed) do not move, but the tip of the handle keep turning about 1/4 or 1/8 of a turn. This will be the correct functionallity of the machine.
If you notice that this last turn is hard, or does not work, you can try the following checks.
1.- Put some soap and water on a bowl, take a tooth brush and try to clean the top part of the "shaft hole", where the shaft of the center bar gets inside the handle. This way the soapy water will lubricate the internal mechanism.
2.- Do the same on the bottom of the tip, where is the "knob retainer cap". Be generous with the liquid, it must enter well inside.
3.- Turn the tip of the handle (close and open the doors) a few times, until the soapy liquid has penetrated well, for a half minute.
4.- Try now another time to turn the handle until its final quarter turn. Now it should work properly.
If after performing this operation, the mechanism is still not working, it is possible that the internal mechanism has some wear, and the piece that allows the last turn is not working properly. This is not a problem when shaving, since there are razors that do not have that locking mechanism (Super speeds), and allow to close the doors perfectly without the need of a internal spring.
After many years of restoration and repair of adjustable razors, we have verified that the mechanisms that locks and the spring inside the razors usually work much better when they are being used by the user and THEY ARE WET, so they are lubricated with soap and water. Sometimes, if the razor is dry inside, the mechanisms are not lubricated and there is a dry friction between their metal components, which makes movement difficult.
In any case, if after these checks your razor still does not work, contact your trusted vintage safety razor restorer.
Remember that you will have to lubricate your safety razor sometimes, take a look to this article, HOW TO LUBRICATE YOUR SAFETY RAZOR
For more information, about cleaning your safety razor, take a look to this article, HOW TO CLEAN YOUR SAFETY RAZOR