The 531 and 535 were the first Tektronix scopes to accept plug-ins. It was introduced in March 1954 simultaneously with the Type 535, and superseded in 1959 by the Type 531A, which has more convenient controls and a slightly faster (15 MHz) vertical amplifier.
In serial numbers 101 to 7600, the 531's L-C delay line is before the vertical amplifier. In serial numbers 7601 and up, the 531's delay line is between the vertical amplifier and the CRT. (What was the reason for this change?)
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.