At the time of introduction, two 'hollow state' devices the MHSVT contained were considered exotic. The first, the 85A1 voltage regulator is a neon filled reference two electrode device which serves to provide the MHSVT with it's reference voltage stable in normal useage to 0.17% variance and over it's entire life. stable to within 0.5% of nominal value. The second is the DG7/5 cathode ray tube which is an electrostatic medium persistance tube . What is interesting is that this CRT is used in this application purely as a voltage indicator, measuring various currents by means of voltage drops across resistances brought into circuit by the punched card valve selectors. No need for a timebase here you see.
With the passage of 60 years or so, opinions change and now we consider a CRT and a neon regulator rather passe and instead become animated if not rather excited by the EL37 beam tetrode that a MHSVT also contains - more on this desirable audio valve in a future blog entry.