Complex sentence: Many of the non-alphanumeric characters were positioned to correspond to their shifted position on typewriters; an important subtlety is that these were based on "mechanical" typewriters, not "electric" typewriters.
Split into simpler sentences: Many of the non-alphanumeric characters were positioned to correspond to their shifted position on typewriters. An important subtlety is that these were based on "mechanical" typewriters, not "electric" typewriters.