Duplotang

Does anyone know at least when they started adding "(c) LEGO Group" to the bottom of the bricks? Or when Lego group was founded? The best page for information on this is https://www.leggodt.nl/items/lego/date-logos.php but this page does not go into this either.

That's an option I haven't even considered... but yeah possibly. If there is a way to figure out the approximate age I would prefer that though.

I have a lot of old duplo bricks and want to figure out their approximate age since I'm a bit worried that old duplo bricks might contain unsafe levels of Cadmium after reading reports like this https://tamararubin.com/2019/05/vintage-1970s-legos-test-positive-for-a-high-level-of-a-known-carcinogen-how-can-i-tell-if-my-legos-are-safe/ I have not found any site explaining how to determine the approximate age of DUPLO bricks (some discuss regular lego but not duplo), or just if they are older than ~1990? (when Lego supposedly switch to Cadmium free plastic colorants). Any tips on determining the approximate age? I see some old bricks have the ©LEGO group logo on the back, some says LEGO A/S, some says just ©LEGO and some has no text at all. Also there are several other differences on the back side of the bricks (see images at link at bottom) but I don't know if that can be use for figuring out the age? For reference the yellow brick is a new brick and the other are somewhat old but if they are from the 70s, 80s or 90s I have no idea. Note, this was also asked at https://bricks.stackexchange.com/questions/18852/determining-the-approximate-age-of-old-duplo-bricks but I didn't get any replies thus a repost here.