Well from the parts manual looks like to different boosters are used>>
brake booster - 1990-1991 10097697
brake booster - 1992-1995 10165315 BOOSTER, BRAKE (AC DELCO # 178-523)
Might have something to do with the style of brake fluid reservoir(s)... MY90&91 look like this>>>
And then post MY91 switched to a single brake fluid reservior.