> I believe this patent is already expired, given that the longest patents (utility patents) only last for 20 years.

It expired in 1994, which was 17 years after the issue date, because the change in U.S. law to implement a 20-year patent term hadn't been enacted then [1].

[1] http://en.wikipedia.org/wiki/Term_of_patent_in_the_United_St...