How can I date my eames fiberglass DAX chairs?

I just picked up a wonderful set of 6 eames cream with columbia blue covered DAX-1 chairs and was curious how old they are. I know most of the other Herman Miller chairs i've owned have a Herman Miller stamp on the bottom and some have a stamped date, but my chairs have neither...only a small long rectangular outline of where a paper label once was. so, my question is, 'How does one go about dating a fiberglass eames for herman miller chair that doesn't have a label or a stamp?' thanks Jeremiah

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