The Digital Equipment Coropration (DEC) PDP-7 is an 18-bit minicomputer introduced in 1965. It is the third of their 18-bit machines, and shares a similar instruction set architecture as the PDP-4 and the PDP-9.
Option 31 is a Tektronix RM503 oscilloscope for generating XY raster plots with 1024 x 1024 bit resolution. It is interesting that they didn't employ the RM564 storage oscilloscope which was introduced three year's earlier in 1962.
This description and illustration comes from the 1965 PDP-7 brochure, courtesy of the Centre for Computing History. Click on the image to view a PDF of the full brochure.
This photo of a PDP-7 under restoration is courtesy of Wikipedia from the PDP-7 page [External link] and shows the Type RM503 oscilloscope in the center rack.
The PDP-7 has significant history as Ken Thompson wrote the very first Unics (i.e. UNIX) system in assembly language on one in 1969.