Well, a part of the mystery is solved, but more investigation to come….

The indicator is an Anti-Icing Fluid Rotameter flow-meter, Type NW, used to titrate anti-icing fluid to the propellers of a dual engine aircraft. Looks to be vintage 1930-40s. It was made by Fischer and Porter of the US, perhaps under license from Rotameter in the UK(?). The sales literature shows an image of a DC-2 or DC-3, but makes no claims that it was used in that aircraft. My question, however, remains unsolved conclusively: What aircraft used this instrument?

You can see the Rotameter and other information at http://aeroantique.com/products/rotameter-anti-icing-fluid-flow-indicator-fischer-porter