[mjevans]

Source: I vote in King County (of which Seattle is a member city and probably uses the same process given the same elections office).

The Mail In Ballot procedure here uses a double-envelope system.

The inner envelope has the vote (with a matching identifier tab pulled off). It is supposed to contain only the vote.

The other envelope contains the inner envelope and this envelope is marked with the address of the voter, and a legally binding contract (signed by the voter and/or representative witnesses if they are unable).

My belief is that the validation process for these ballots includes multiple stages and probably blinding, wherein, the outer envelope is authenticated as a registered voter and that it contains something likely to be a vote (there's a small viewing circle). This would then be stripped and the trusted inner envelope, still sealed, added to a tabulation bin. Said tabulation bin would then be counter later in an anonomized manor.