Adam Back replied the next day, confirming that Satoshi’s draft Hashcash citation was correct and pointing him toward Wei Dai’s b-money proposal as a second piece of related prior art. Back’s load-bearing remark — the parenthetical that drove Satoshi’s next step — was:
the b-money idea is just described concisely on his web page, he didnt write up a paper.
That sentence redirected Satoshi to a web page rather than to a peer-reviewed paper, which set up the citation-year question Satoshi would put to Wei Dai directly the following day (see the 2008-08-22 Satoshi → Wei Dai email). Within hours Satoshi also wrote back to Back himself acknowledging the b-money pointer (the third email in the chain).
Adam Back’s pointer to b-money two months before the whitepaper is one of the primary-source observations underpinning the cypherpunk-independent-arrival analysis: Satoshi reached the design space without prior cypherpunk-community immersion, and learned of b-money only via this referral.