Reporting Plagiarism in Older research
The perception of plagiarism has evolved significantly along the years, and practices that are not tolerated nowadays were instead not unheard of in the recent past. I recall seeing several papers from the 1980s and 90s replicated almost identically in different venues, without any cross-citation or copyright notice.

Moreover, bibliographic searches in the past were not so easy, and the authors of the most recent work may have found the same result independently and missed the older work.

Since almost thirty years have now passed from the publication of the second article, I'd ignore the matter: nothing really useful would come out by bringing it up.