Scott Joplin in this case kind of gets repetitive, so bear with me through this one. The general form for measures 69-84 is:
mm. 69-76 77-84
a b a c
You will notice that both A and A' are both 8 measures long, making this a regular parallel period. Additionally a, b, and c are each 4 measures long which means further demonstrates the regularity of this excerpt. Also given that there is a repeat around this whole section, it is a double parallel period. o. Awesome.
A quick rundown of the chordal analysis results in the following:
Key of Eb:
mm. 77-78: bVI
mm. 79-80: V64
mm. 81: IV7
mm. 82: V864
mm. 83: V753
mm. 84: I
In measures 77 and 78, the mode mixture is represented by the bVI chord. This chord is spelled Cb - Eb - Gb. One really cool thing that Joplin does is that he starts hinting at this mode mixture before it even happens. He takes an enharmonic spelling of one of the notes in the bVI chord and slips it in as an embellishing tone. In measures 69-76 there are two outliers that I found, one C# and one F#. I think F# is the culprit because it is enharmonic to Gb (apparent in the bVI chord). In this way, Joplin hints at the mode mixture that is to come. The reason it is F# is because resolving Gb to G is not appropriate and it must be F#.
The contrary motion in the voice leading of the resolution:
bass: Cb - Bb 6(b) - 5
sopr: Gb - G 3(b) - 3
is also interesting because the bass on the & of 2 repeats the G natural that resolved in the soprano line. Lastly, there is a 64 chord in measure 79 that functions as a cadential 64. We all know about the V64-53 well this one I think eventually goes V64 b64 53 I . Cool huh?