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1955 ish...
My best guess on the earliest H logo (often with no other markings) is 1955 ish... But Woody or Lunchbox know more than I do.
The H-base is likely...
The H-base is likely introduced in 1955 (solid metal).
Shells with embossed HM logo are produced later then 1957. First with only the 'H' (+ patentlabel and medaillon) and later with 'Herman Miller'.
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