Super Mario Bros. is a near-perfect recreation of the levels in the original Super Mario Bros., though there are a few inaccuracies. It was presumably created as the game was developed since World 1-1 is seen in most pre-release screenshots, though it's also possible he created the rest of the levels after all the Mario entities were implemented; either way, it is still the first mappack created for the game.
In Mari0: Special Edition, you cannot view this mappack normally. This is because its settings.txt is missing from its folder, but you can fix this by adding one via your mappack folder or through modding.
Additionally, the Minus World is missing. This can be remedied by editing 7-2. Hook the pipe at the end to Main, then, in your mappack folder, rename it to "M-1.txt".
These problems are likely the result of Maurice carelessly using the 1.6 mappack converter which ignores those files (and any other non-level file).