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 It was introduced in 1954 simultaneously with Type 535, and superseded in 1959 by Type 531A, which has more convenient controls and a slightly faster (15MHz) vertical amplifier.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.