Continuity mistake: When Morty is giving Jerry directions to the scuba diving site, The Pen alternates between being in Morty's hand and on the table between shots.
Revealing mistake: While Jack is leaving The Apartment after giving Jerry The Pen, you can see they have put a fake background outside the front door. What's more it is not even that realistic. It looks like a forest with a river in the middle. The Seinfeld's Florida apartment is situated amongst many other apartments around it.
Answer: The name is Delores. He shouts that name at the end of the episode. Then in a later episode, "The Foundation", he runs into her. And he repeatedly calls her Delores.