The bottom line is that the "description" tag of the 9735 Zap Receipt seems to contain the json string of the 9734 Zap request. The "pubkey" field of the Zap request is the sender of the zap. However, at least some of the wallets seem to delete the "amount" tag of that Zap request; and so the amount of the zap is lost.

I'll write up a blurb about this for NIP-57 and do a pull request. My luck getting PRs into the main-line has not been great, so we'll see.