The era should be dynamic; when an ethereum:0x44b28991b167582f18ba0259e0173176ca125505 is sold, not an NFT, the old era should be destroyed. The vacated space is filled by the one with the next lower ID.
influenced by the culture that made sequence matter - the uPeg ID is the defining signal of provenance + origin.
with @OpenSea, a new trait arrives: Era 🦄
every 1000 IDs is a generation.
Era 1 // IDs 1-1000. Era 2 // IDs 1001-2000 // etc
coded into the contract. immutable.