Some light hints that may point you in the right direction:
Hint 1
The assembly is a part of a larger machine.
Hint 2
Several assemblies are meant to move side by side in unison.
Hint 3
The square-ish shaped piece is plastic that will wear down and be replaced.
Look up a Pitney Flowmaster if you're curious for more.
Collates the inserts into stacks, aligns and opens the envelopes, pushes the insert stack into the envelope, folds and seals the flap, flips the envelope, applies a stamp, prints addresses or data, reads the barcode, ejects any flagged envelopes, and finally stacks them all in a neat row.