If you get the "Sorry, the file cannot be displayed. There seems to be a technical issue ..." error when trying to view an image, a workaround is to click on the cog icon in the upper right side of the media viewer, and then select "Disable Media Viewer". We are aware of the problem and are looking for a solution.
The Type 531 is a 10MHz oscilloscope that accepts letter-series and 1-series plug-ins. It is exactly like Type 535 except it does not have the Delayed Sweep (dual timebase) feature. The 531 and 535 were the first Tektronix scopes to accept plug-ins. It was introduced in 1954 simultaneously with the Type 535, and superseded in 1959 by the Type 531A, which has more convenient controls and a slightly faster (15MHz) vertical amplifier.
There is also a rack-mount 531A, Type RM31A. The RM31A has a sliding chassis so that the scope can be serviced without removing it from a rack. The 532 is similar to the 531, but designed to be more reliable at the expense of slower risetime.