The SteamPeaks account does not own Letters - Artbook DLC, so no manifest of it has been read and there is no size to measure. The Depots section shows the sizes Steam reports for each depot; the history starts the day a free licence or a key adds the game to the account.
No silent builds: every build of Letters - Artbook DLC on an unprotected branch in the last 60 days had a patch note or an update event within a day of it, is under 24 hours old, or has not been judged yet. The hourly check fills this in.
The SteamPeaks account does not own Letters - Artbook DLC, so none of its files are in the index and nothing can be compared until a licence adds it. 2,429 games are indexed today.