Well, 7ths chords are everywhere (rife) in my studymaterial ( 9ths ?..must look)
Triads with their inversions seamlessy playing is the first step then.( its good for composing ) ( but for keyboardplay i do have a oversight of chords in the best possible finger positions, so no need to know all inversions of triads)
The 7ths are rather easy to play in AI setting and in multi fingered you must grab the whole block 7th chord with 4 fingers.
Not to mention the inversion from the 7ths chords, there are a lot of them, but the studymaterial (course 1) shows how to play it
I do see a example here in lesson 13 a D7 chord in a inversion ( find out yet? )(there is chord tutor on SX600)
Note: all chords in this course (nr 1) are probably arranged in a way that a playing a chord sequenze goes on the most easy way?
In another keyboardcourse(nr 2) in book 1, the C7 is played with 2 fingers, probably done this in order to master more easy the melodyplay? ( this keyboardcourse (2)is based to use the one fingering chords.
But playing first one-fingering chords as exercise in this course (2) and again with fulll fingering chords?
Enough to practice ..
Jan