Not sure if this question has a defined answer, or is a matter of opinion. I have a 1941 piece of radio equipment I am thinking about restoring, and I told my dad it was "prewar". It was manufactured before December 7th 1941...so it was before the war in the US, but not prewar overall.
Yes, it is both appropriate and common to refer to things Americana before December 1941 as prewar.
True, the war was raging since 1939 (or much earlier, 1937, if one considers what was going on in Asia) and while not directly involved in the hostilities, America was a participant indirectly via materiel aid to the UK, and volunteers flying in both China and with the RAF.
Life in America, including production of goods, went largely unaffected by "Europe's War." Cars, boats, radios, stoves... these goods were churned out as usual. All of that changed after December 7th, 1941. Manufacturing was shifted to wartime production. Automobile factories began churning out tanks and planes and suspended making cars for the duration. Wartime restrictions were placed on consumer goods, as can be seen in this report from the Economic History Association.
Thus, from an American perspective, regarding its products and culture and politics and economics (and other broad measures), it is appropriate to refer to anything Americana before December 7, 1941 as prewar. In fact, there are many such supporting references to be found, such as:
Unemployment remained in double digits even as inflation in 1941 approached 11%. Of its annual $9 billion budget, the federal government borrowed $6 billion, or nearly two-thirds of all receipts. Americans continued to struggle to make ends meet even as the great events of December 1941 were unfolding around the world. Prewar America, as described by Shirley, bore little resemblance to the postwar super power America would become.
Source: Human Events article on December 1941: The Month that Changed America Forever
This Hemmings article about a 1941 Fargo half-ton truck is titled 1941 Fargo 1/2 - ton pickup. Pre-war pickup was produced in Ontario, for the Canadian and overseas market (Fargo was a Chrysler subsidiary).
Here is a commercial product advertised as 1941: Prewar Style.
Here is a model train hobbyist article titled A Gilbert Prewar American Flyer 3/16" O gauge [offered in the 1938-1941 catalogs] pictures and information thread.
This Fairfield Collectibles die cast shopping page advertizes Prewar Classics and includes a 1941 Coca-ColaTM Chevy box truck.
In the context of American culture, there is extensive support for calling anything manufactured in or by America up to December 7, 1941 prewar merchandise.
Context; context; context. The aim is to communicate, accurately; so be kind to your dear reader.
Anytime the context of a statement is entirely American, it is clearly correct, even most correct, to refer material produced prior to late fall 1941 as pre-war.
However anytime the context is not entirely American, such as with Lend Lease for example, the phrase pre-war should reference the declaration of war by Britain and France in September 1939.
More complex would be contexts that specifically reference the U.S. and U.S.S.R. Here the two declarations of war are only six months apart. However the aim is the same - to communicate clearly to dear reader. If U.K. and Commonwealth (and France and Poland as applicable) are in the context then take more care.
Remember that the phrasings pre-1939 and pre-1942 are available as disambiguations.
'prewar' becomes important when you consider the context in which wartime production functioned. In the case of a radio, it is almost certain that it's prewar, as consumer radio production essentially ceased during the war.
During WW2, most of the higher end consumer goods were discontinued in favor of war production. A radio would have been the equivalent of a large widescreen TV back then.
Each one of those hundreds of thousands of tanks and aircraft built during the war needed... a radio. Any company that could make a radio was given a government contract to make military radios. Any company that could make the components for a radio: tubes, capacitors, resistors... was selling all they could make to companies making military radios. Compounding this was the high employment for war production, coming out of the Depression. A lot of people were now making money, but there wasn't a lot to buy with the money they were making.
In short, a consumer radio actually made during the war would be extremely rare, only on the possibility that the maker might have finished up a few out of parts lying around. Wartime production began to relax in late 1944 and into early 1945, when it became apparent that both remaining Axis powers were definitely losing, so a few high end consumer goods began to trickle out.
I suggest you make an effort to pin down exactly when that radio was made, if possible. No, I'm not sure how you can do that, other than maybe examine the chassis for a date. Some of the components such as tubes or electrolytic capacitors may also carry a date code.
If it was prewar, which is likely, it is interesting. If it was made during the war, especially in the peak war production range of late 1942 to late 1944, it becomes very interesting.
Also, be very careful if you're thinking about applying power to it. It's possible to revive some of the internal components by starting with a very low voltage and slowly increasing it, over a period of days, but you need to do a lot of research before you try that.