Most likely dirt, oil and grime from the fingers and palms of the mechanics who've been using it for so long. That in itself is testimony to the C64's ruggedness. When I worked at a computer tech store I sometimes got PCs that had gotten messed up by cigarette and pipe smoke from users who were smoking while they worked. Smoke accumulating over two or three years. Makes this Commodore still working after a quarter century even more impressive.
Honestly, that looks more like the paint/finish has just been worn off from decades of touching rubbing etc.