Sechs

The front cover lists this as SECHS and there is a line on the front cover "Jetzt ist alles gut"

The "Sechs" (german for "six" or 6) refers to a continuation of numbers hidden on earlier vinyl releases of Die Tödliche Doris. The statement "Jetzt ist alles gut" (now everything is fine) is an answer to the question "Wie geht es dir jetzt" (how are you right now) from the previous/companion album Unser Debut.

This is actually the second part of "Die 5. Unsichtbare LP". The first part is the LP Unser Debut.
On both LPs the individual tracks feature the exact same length. Both albums (well, all three to be exact) were conceptualized at the same time, but recorded at different locations and at different times. All compositions, lyrics, titles complement each other, but can also be listened to and enjoyed on their own. If you play both records at the same time you will hear "Die 5. Unsichtbare LP" ("The 5. Invisible LP").
Also the artwork of both releases correspond with each other, each features hints and elements that were picked up on the later release.

This was only made public on November 8th, 1986 via press statement and Din A1 poster called "Schmeiss die Krücken weg" (throw away the crutches).

In Japan, both albums were released together as a double-album (triple album so to speak) Die Tödliche Doris - Unser Debut + Sechs .
Later the "Unsichtbare 5. LP" was materialized, first as a CD in 1993 as Die Unsichtbare 5. LP Materialisiert Als CD / The Invisible 5th LP Materialized As CD and in 2015 on vinyl as Die Unsichtbare LP.