The Mullard Master Test Board (MMTB), introduced in 1935 is in actuality a re-badged Philips Cartomatic GM7629, often referenced as the Cartomatic Mk1. In every other European country it was sold as the Philips Cartomatic but for some reason, Mullard badge engineered it as their own product. Physical examination of the Philips Cartomatic Mk1 and the Mullard Master Test Board side by side reveals identical construction leading us to conclude that irrespective of badging, they were all made by Philips and either supplied fully built or as 'knocked down' kits for British construction and then post manufacture rebadged as Mullard instruments. Curiously, most Cartomatics appear to have a replacement meter which is of smaller diameter than the original, the tell-tale being the diameter adaptor securing the meter to the front plate. Obviously these gave some trouble in use which may have had a bearing on the design of the MHSVT which has a CRT for test indication rather than a relatively fragile moving coil meter.
Here is an image of the MMTB, courtesy of George Bichard - see the card slot and operating handle (20) on the right - does it look familiar?
The MMTB could also function as a multimeter as well. In actuality, the MMTB was a Philips Service Tube Tester (PSTT) with reduced valve testing capability but with multimeter functionality added in. Also introduced in 1935, the PSTT was a professional grade machine used extensively within Philips' own laboratories and service centres. Below, you can see a photo, courtesy of Jean -Yves Loizeau which shows such an instrument in use during the 1930s at a Philips service centre:-
And here you see a PSTT in it's full glory, working away, mercury rectifiers glowing in the rear and four beehive neons burning away on top - I cannot stress how rare a sight this is for to my knowledge there are currently only two PSTT known to survive, one in Philips' own Eindhoven museum and the one pictured here preserved in captivity in France under the care of Jean-Yves Loizeau.
Both the MMTB and the PSTT, like the MHSVT use plug in cards to input valve test parameters, however, whilst all three are similar, none of the three types of card are compatible with each other.