Awesome, thanks for this response!

That's Hive account@null and Hive account@temp - all three accounts are part of the initial bootstrapping configuration of the blockchain.

Hive account@null makes sense. Does Hive account@temp still have a special meaning?

The transactions to create accounts via mining were proof-of-work operations, pow, and later pow2 - the resulting rewards could be either vested to existing or new accounts

So, to create a new account, the actual operation was vesting? A miner used his rewards to vest to a non-existent username?