How to get out of the screen these specs or dirt accumulated over so many years.
There are escalating possibilities to clean the screen - depending on the difficulty in getting out the crud.
The first strategy is to try to move it out. For this you will need a keyboard cleaner - these sprays of compressed gas. It can easily be found in amazon or aliexpress.
The first way of using it is through the two open places in the back of the calculator. If that doesn't work, you will need to open the calculator. Beware - opening a calculator may mean that you break things that will require a full repair. You need to decide if it's worth trying.
To open the calculator, you need to unscrew the four screws that hold the unit together. These are below the 4 rubber feet. To take them out, use a small flat screwdriver. I usually start from the inner part. In the lower feet, the screw is in that area and it is easier to lift the feet with the attached adhesive film. In the upper feet, the screw is more or less in the center of the feet. Make sure that the film is lifted together with the rubber feet and not destroyed. Put them aside in a place where they don't adhere to your hands or sleeves.
Now unscrew carefully the screws. These are there since 30 or 40 years, and are self-threading - so be careful and unscrew them slowly so that the brittle plastic doesn't break. Put them aside with a magent so that they do not disappear.
With the screen facing down, pull up the back half case. Now you should be able to see the back of the screen. You need to try now to use the cleaner through the sides of the screen.
I usually stop there. If you really want to clean it more there are two options: bending the screen cables or unsoldering the screen. Both put the screen on risk - your call!