This is the system panel from an IBM System/3 Model 15D.

The black square at the lower right is an indicator which displayed a two position halt/status code. The example on the left shows the code "EJ" which was a normal end-of-job indication. There were hundreds of codes used, referenced in the Halt Guide. On the original Model 10, this was the only way the system communicated status to the operator. Later, a Selectric typewriter (machine 5471) was available as an option where the system could output response messages. The Model 15 included a CRT screen and keyboard for operator interaction.