Theoretically, maybe you could do it over letters. Just like you could run a computing algorith with horsemen and flags in place of a CPU (like in the scifi novel, Three Body Problem).

Practically, you are talking nonsense and it's patently impossible. You cannot run any crypto currency with a system of letters even if you're transporting them with the full infrastrcture of the modern world with it's airmail and diesel vans.

Try and do it with letters delivered on foot. It is actually impossible.