Here are some ways to estimate the date of manufacture of a radio.
Cabinet style
Rectangular box with separate speaker (1923 – 1930):

Cathedral cabinet style (1930 – 1934):

Tombstone or Skyscraper cabinet style (1933 – 1938):

The tuning dial
Keyhole dial aperture (1930 – 1934):

Semicircular dial (1932 – 1934):

Full Circle dial (1934 – 1940):
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Pilot 403B (USA 1936)
Rectangular dial (1938 onwards):

