And that is what he did. The Democratic Republic of Germany (East Germany) granted him a transit visa and he bought a ticket to East Berlin. He called us with his arrival date and time at Berlin's Schoenefeld airport, located in East-Berlin.
And here is the loop-hole that helped us get him into West-Berlin, which would also mean into the Federal Republic of Germany (West-Germany).
I was able to enter East-Berlin officially for a day by just showing my ID. I then took the Underground (U-Bahn) and the city train (S-Bahn) to Berlin Schoenefeld Airport. As the airport was an accessible zone for everyone, this was no problem. My husband did not go with me, as he always had trouble crossing over any border with his Iranian passport.
My father-in-law in the meantime passed through customs in Schoenefeld without a problem with his Iranian passport because he had a transit visa. I was able to meet him at his gate.
We then proceeded back to the S-Bahn, changing to the U-Bahn, until we reached the station "Friedrichstrasse" in city center. Here we had to pass through another East control as we wanted to exit East-Berlin. I passed through after a bit of questioning, but without any problems. And my father-in-law was able to go though this control with his passport and transit visa. We did not have to leave the U-Bahn station, we just changed platforms and got on a West-Berlin train (the purple line shown on the map below). This was the loop-hole - there was no control into the West. Yes, at that time police were patrolling the platform, but they were looking for smuggled goods, like cigarettes and alcohol that were being peddled from the east into the west. They did not have an official border control at that U-Bahn station, so we just took the train and rode home. And that was how my father-in-law came to West-Berlin, applied for asylum and later on for a visa into the U.S.
|Berlin underground lines (Source)|
On the right of the map, you can see the grey line, marking the border to East-Berlin. The purple line which we took, ran through the East, but only stopped at that one station, Friedrichstrasse.