The following is an excerpt from philcoradio.com by Philco historian Ron Ramirez regarding the origins of the Tropic series of radios. New Zealand models often differ from the US models most likely due to our import restrictions meaning the NZ versions were (designed and) built here - often differing from the US counterpart either a little, or a lot.
In June 1938, Philco introduced its new 1939 model line. For the first time, the company not only had its regular line for the USA market, but also a specialized lineup designed specifically for export to the tropical regions of the world.
Philco called the new line Philco-Tropic Radio. Philco described the special construction of the Tropic models in their 1939 Philco RMS Yearbook as follows:
The Philco-Tropic radio is particularly recommended for locations where super reception of short wave is necessary and where the radio and the cabinet are exposed to extreme conditions. The receiver is especially constructed to withstand decay, spoilage and deterioration caused by extreme conditions of humidity, heat, salt air and cold; and to stand up under the most severe tropic weather conditions.
The cabinet is treated with a special sealing compound which protects it against moisture and heat.
The chassis is heavily plated, making it impervious to salt air, rust and corrosion.
The various parts, such as coils, condensers, chokes and transformers, are treated with special wax that will withstand very high temperatures. In addition the wax is treated with chemicals which repel rodents and insects.
Philco continued to describe its Tropic line of radios in its 1940 RMS Yearbook in a similar fashion. But by the 1941 season, Philco had ceased mentioning any sort of "special treatment" having been given to its Tropic radios.
All Philco-Tropic radios were built for operation on either 115 or 230 volt lines. Some small Tropics operate on either AC or DC, while larger Tropic models were designed for use on AC only.
Philco-Tropic radios are noted for their emphasis on short wave coverage. Some of the better Tropic models include several short wave bands, designed for easy tuning of short wave stations.
The New Zealand Tropic models appear to often have a chromed chassis and while other NZ models may also have chrome, mostly they were painted blue.