CaseIH records indicate that the change was made approx. April 1, 1989. Since their production year begins around November 1 every year, I guess you could call the Cummins-engined Axial-Flows 1989 1/2 models.
We have a 1989 Case-IH 1660 with the Navistar (Cummins) engine. Oh man, this baby runs great. I joke that it whistles while it works! Most of them had some issues, but they supposedly updated them all so you should check on that first. I have heard some of the oil pumps were weak too. I don't know but ours has about 3200 hours on it and it is one sweet motor! The rear main just started leaking this year so we are going to fix that but other then that, check for electroysis (with multimeter) and add diesel and change oil and you should be set.