The Pioneer SX-x3x models along with the flagship SX-1010 were produced from 1974-1976 and are considered collectable for their performance and looks. This line is unique in the entire SX series for the inclusion of various function buttons through the glass. Most models included an FM Muting button with the SX-1010 having FM Muting and a Dimmer button for the dial lighting. The labels for these buttons are stenciled right on the glass and for the most part have held up reasonably well, but the careless use of harsh glass cleaners or aggressive scrubbing will certainly damage the lettering. Fortunately, there are some options depending on the level of damage.
If the damage is minor with only a few letters partially affected it is possible to do a passable repair with a steady hand, some magnification and a fine point gold Sharpie. But for more significant repairs it is easier to start from scratch and re-stencil the glass or replace the glass entirely.
If the glass is in good shape and you would like to keep it, then your best option is our Glass Stenciling Kit. Check out this video to review the steps and level of difficulty.