I am skeptical of the assertion that this game has only 6 unique levels of content.
The block layout is governed by $00A1. (The background is $00A0.)
Here is how the game uses this to load a level:
398: A5 A1 LDA $A1 A:00 X:00 Y:00 SP:FB P:27 nvTbdIZC Cy:519250913
A39A: C9 10 CMP #$10 A:06 X:00 Y:00 SP:FB P:25 nvTbdIzC Cy:519250916
A39C: 90 04 BCC $A3A2 A:06 X:00 Y:00 SP:FB P:A4 NvTbdIzc Cy:519250918
A3A2: 0A ASL A A:06 X:00 Y:00 SP:FB P:A4 NvTbdIzc Cy:519250921
A3A3: AA TAX A:0C X:00 Y:00 SP:FB P:24 nvTbdIzc Cy:519250923
A3A4: BD F8 A3 LDA $A3F8,X * A:0C X:0C Y:00 SP:FB P:24 nvTbdIzc Cy:519250925
A3A7: 85 10 STA $10 A:A0 X:0C Y:00 SP:FB P:A4 NvTbdIzc Cy:519250930
A3A9: BD F9 A3 LDA $A3F9,X * A:A0 X:0C Y:00 SP:FB P:A4 NvTbdIzc Cy:519250933
You can see that the game compares $A1 to #$10, indicating that the game loops at 16 stages.
Actually, if you are over 16, the game does this:
A39E: 29 0F AND #$0F A:10 X:00 Y:00 SP:FB P:27 nvTbdIZC Cy:519250920
A3A0: 09 08 ORA #$08 A:00 X:00 Y:00 SP:FB P:27 nvTbdIZC Cy:519250922
So the last 8 levels are the ones that are repeated. It promptly discards the value of $A1 after this point. I'm not aware of any other variable affecting level layout.
And anyway, level 7 is pretty clearly not just a shifted version of the previous levels.
EDIT: Also, there doesn't seem to be anything special about the initial 6 levels. You could say that every level is 'procedurally generated' content.