The T3 series is a set of 8 disks describing the 8 ages of Man. This is disk 1, and there’s what appears to be a child’s head, and it looks to be sucking its thumb. It fits.
The letter in the corner is “A” – with another seven disks perhaps there’s a word to be spelled out, but the final disk has, as we’ve seen, “CO2” instead of a letter. Perhaps this “A” is a gas or element instead – but there is no element with the symbol “A”.
(Image: © Copyright Trevor Wright and licensed for reuse under Creative Commons Attribution-ShareAlike 2.0 License.)
Here’s the original diagram: