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Based on the style of the letters and use of the English alphabet, I suspect the Early Showa period, most likely a decade before the war (戦争前). It would be the 1930's in normal year format.
Thanks for that. This is really helpful.
*I live in Japan so I could research the photograper if requested[/i]
There's no need to research the photographer, because there's no photo attached to the cardboard. It was removed some time ago and the cardboard was 'recycled/ re-used' for other purposes. I just wondered how long ago this could have happened. I realise that this recycling could, at least in theory, have taken place any time between the 1930's and now, but I really was hoping it happened before 1950. More recent would have raised extra questions.
I'm glad I posted the picture. Never expected that the characters by themselves told such a story.
Thanks again for your explanation.