One of several makers of this type of once-common unit, the Vickers appears to have been of the superior kind where the dials were geared together and so as accurate in their conversion as possible.
Once the definition of an inch had been set at 25.4 mm, conversion from one to the other became possible (before that, the definition meant a long list of further decimal places). The Vickers units were a serious attempt at marketing the retrofit idea with no fewer than nine different diameters of dial, fittings for more than 60 machine tool makers (from UK, European and U.S.A. makers) and over 400 designs available. The dials could be fitted not only to the top and cross slide screws of lathes and their carriage handwheels - and the table and head-feed screws of milling machines - but also to the feed screws on machines unrelated to engineering. Hence, the potential market was huge - though it's difficult to know if a machine has a dual dial by Vickers and they appear not to be marked with the maker's name. One design flaw in some of the Vickers unit was the absence of a cover to hide that section of the dial not being read, a flaw that could so easily lead to an operator misreading the setting. This failure was overcome by another English maker of dual-dials, Gamet - a Company better known for its high-precision roller bearings used on Colchester and Colchester-Clausing lathes. The Gamet units employed a snap-in-out sliding cover and appear to have been used extensively by Colchester with, so far, no publicity literature coming to light showing that the units were offered to other makers. Gamet held, unusually, three different patents - demonstrating that a considerable amount of effort must have been put into developing the unit - although it appears that only one of the trio made it into production, that patented by a Mr Stanley Hill.
Almost certainly acquired through licensing (the patents were held by individuals) they were GB 1,193,831 by Charles Sindall (believed to be an American) in 1970 and two for which data is hard to find: GB 1,264,994 by Waplington and GB 1,463,732 filed in 1974 and issued in 1977 by Mr Stanley Hill.
Despite this, the Sindall patent also incorporated a sliding cover, though the internals were arranged differently.
The catalogue extracts below explain how the gearing was arranged and the mathematics of the ratio conversion - though, interestingly, the parts drawing omits any details of how the gear train was arranged - perhaps it was a case of "patents pending" or even the inability to secure one..