I have four of these mainframe style oscilloscopes with a number of plug in units. The 560 series was something of a transition from 500 series to the semi-conductor age. Unlike the 500 series mainframes that included the Y amplifier, X amplifier and timebase(s) on board, the 560 mainframe was basically a super high quality indicator unit with regulated HV for the CRT and precision regulated supplies for the plugins. It did still include the fine Tektronix amplitude calibrator. And so, it accepted both X and Y plug-ins; a wide range of plug-ins, mostly hybrid tube and transistor, was available that included tunnel diode triggered dual timebase units. So the trade-off was performance versus flexibility and of course size and weight! The bandwidth at 10 MHz was limited, especially in comparison with the 545 and 547 yet enough for many applications. 1 GHz bandwidth was served by a version (567) that could accept sampling plug-ins. Perhaps most notably Tek introduced a truly versatile storage oscilloscope with type 564. I think that Hughes beat Tektronix to the market with their 104D storage scope but it was single channel with a very limited 250 kHz bandwidth, also the storage tube was fragile. Here is serial number 13502 fitted with a 3A74 4 trace Y amplifier and the most basic 2B67 time base.
Here it is again showing the OS-34 / USM-32 time base waveforms:
The 561A mainframe contains just the CRT, the calibrator and the power supplies. Unlike the 500 series mainframes, the time base is also plugged in and there is no Y deflection amplifier, that too being part of the plug in. It has a maximum bandwidth of 10MHz which even in its day was not stellar. However, it is much smaller and lighter than the 500 series and extremely flexible. An example of that flexibility is that third parties could and did produce specialised plug ins such as the Nelson Ross -021 Spectrum analyser.