Yep. That was the prefix area... You could do something like SET NUM ON and it would show the line numbers (much nicer than the =====) or even hide the entire area. Was a good match to a 327X terminal. Clearly built by programmers for programmers in other times.

Despite it's reputation IBM had some amazing people (e.g. the name Mike Colishaw comes to mind) who apparently had the freedom to do some cool stuff. There was source code for everything (you got source code for your OS) ages before anyone knew what "open source" was and there was a vibrant community of sharing code between various academic sites who ran IBM mainframes.