WHBM
"Unless it's been re-engined in recent times, this 737 airframe had JT8D-9A engines."
Yes, I believe that's correct. It certainly had the -9A for the first 20 years of its life with PWA and Canadi@n.
While all the 737s being produced at the time it was built (1975) were designated as Advanced and incorporated all the relevant aerodynamic mods, operators still had the (presumably cheaper) option of specifying the -7/-7A/-7B/-9/-9A, if desired, rather than the uprated variants.
Whether or not that turns out to be a factor in what happened remains to be seen.