I have four of these mainframe style oscilloscopes with a number of plug in units. 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.