https://dl.acm.org/doi/pdf/10.1145/103085.103097 (
https://dl.acm.org/doi/pdf/10.1145/103085.103097)
As an interesting note, the April 1991 article was released when NEXTSTEP 2.1 was shipping, and states that NEXTSTEP 2.2 was expected to be released in the fourth quarter of 1991.