The Tektronix 5000-series was a range of lower-speed modular scopes made from 1971 to 1991, covering target markets like mechanical or biomedical instrumentation, or audio. They complemented the high-end, modular 7000 series of lab instruments.
The 5000-series plug-ins look mechanically similar to the 7000-series. They have the same front panel dimensions but the 5000-series plug-ins are shorter, and the mainframe interface is not compatible, it is even using different supply voltages. However, there are some plug-in modules that come in almost identical 5000- and 7000-series versions, e.g. the 7S14/5S14 sampler or 7CT1N/5CT1N curve tracer.
51xx (slow) mainframes
54xx (fast) mainframes
These scopes are compatible with 5000-series plug-ins.