This is talking about competitions that happened 1980-1981Even if the program in an experimental model used for a championship is not different from the production model, the speed of the central processor chip may be twice as fast -- which can make a huge difference to move choices in crucial tactical situations. Such enhancements, which may be possible under tournament conditions where special cooling equipment has been used and the company's programmers and engineers can be on hand to cope with breakdowns, are not usually possible in production models -- which have to work
in normal conditions in owner's homes and whose price must be kept as low as possible.
The New Chess Computer Book
I figure it has to be one of the earliest examples. According to this guys and girls were overclocking their 6502s and z80s from a very early period.