My guess on the mystery LAX-SJU airline would be TWA, with a 757-231.
jrl767, I'd forgotten about Pro Air!
I was living in Troy, Michigan, and on a whim, I drove down to DET the day they began service (their first revenue flight was DET-BWI-DET), and one of their managers invited me to ride their inaugural flight, as a guest of the airline. All of their employees were very dedicated and kind, but I felt the neighborhood around DET, which has been declining for 60+ years, would probably deter passengers from flying them - unfortunately I was right.