Bought a really old book and found a really old USPS card in it

So I bought an American history book from 1871 and inside was a certified mail signature card. So it turns out that after 140 years, the USPS is tracking signatures exactly the same way but with green cards instead of the manilla cards they used back in the 1800's. I checked with the experts at Google and was told they changed the cards to green so that they don't blend in with the rest of the mail