I've worked on trade-card backs for myself and I've used my very own tradecard backs.
This is not to say, this should be official, but I just don't like the parchment in the first place.
I have some reason for this, and I'll explain:
trade is something generated whenever the first cities are build and last until the end of the game. We could choose for different approaches on the subject.
All trading is communication, and the base of all trading (even before the first trade was ever made) is communication between two nations/tribes.
I would say 'communication' should be the subject on the back of the cards.
Parchment is developed mcuh and much later than the first trade was made. Actually invented/developed when libraries became that important that papyri were too little available. This is in a much later stadium.
In my backs I've chosen for the base of communication: drawing/painting and thus took a cave-painting. In my opinion that's a great option for backs.
We could also choose for a combination. say a mixture of a cave-painting, writing on papyri, parchment, clay tablets, rock tablets
I could try for something, but still I'd prefer just the basis of communication: a cave painting.
It's also recommendable to take a chaotic back instead of a solid color because of the shining through when you hold a card against the light.
Then the name of the game on the back. It's something to choose for, but when we are in this stage, where we still don't have a name for sure 100%, maybe it's better to leave the name off of the back. I'd rather choose for some sort of sign (like the AH symbol)
Maybe a hand is a good sing that tells the origin of mankind. intelligence and creativity. We could put that symbol (in a more stylish form) on more places
