Kodak 'Velox' Exposure Indicator. Model E600 UK Man. 1935 / 1939
These two instruments are basically just dedicated micro-ammeters. Beyond what is said on the meter faces, very little is known about them, or can be found out. They would have been linked to the type 594 Photronic cell which would have delivered a small current proportional to the light falling on it.
They clearly formed part of a semi automatic commercial printer made by Kodak for the burgeoning print shop trade, pre-2nd World War. The later unit sits in a panel mounting ring. The left-most of those shown above was made by Weston Electrical Instrument Co., Surbiton, Surrey. This dates it between 1932 and 1936 which was when UK production switched to Sangamo in Enfield. This is where the right-most unit comes from so these meters must span the 1936 date.
Exactly how the printer worked is speculation. But the Photronic cell must have been in the receptor path with the exposure lamp shining on it through the negative (probably on roll film). The brightness of the lamp would have been varied using a rheostat to achieve a standard (or centred) reading on the meter (more evident in the later of these units). The Velox printing paper would then have received a standard exposure time, with the lamp brightness compensating for variations in negative density.
The cell couldn't stay in the light path or it would interrupt the exposure to the printing paper. The degree of automation again can only be speculated on. The more recent unit would tend to suggest that, apart from the cell position, setting exposure would be a single action. The older unit, with its ten-point numbering scale, would suggest that readings may have needed transferring to another control on the printer.