Well, you found a symptom of the cause (why I recommended you keep looking to find where all that buildup of material was coming from) Unless you're doing heavy driving that burns up clutches quickly, you shouldn't have that much material (either clutch disc or metal shavings) build up like that. That missing Rivet (and I'm just spit-balling here) was probably causing uneven wear on both the pressure plate and the clutch disc, and causing that build up on the input shaft. Glad you figured it out (fingers crossed).
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