I have no idea what passport he used, but PLEASE do not repeat the falsehood that US passport holders were restricted in any way from traveling to Pakistan in 1981 (or any other time from 1947-2010), because it's not true.
I didn’t say that because I know it is not true. He could have gone there, if he had been able to obtain a visa, with a US passport; if he had had one. But he didn’t have a US passport until he became a senator, I think an Illinois senator. So, since he didn’t have a US passport, it must have been from another country; they wouldn’t have let him in to Pakistan and Indonesia - and IIRC he may have visited Hyderabad, India - without a passport of some kind or another.