Thanks for checking on this.
Sadly I haven't been able to find an eng number. Have looked through the 'R' fastener reference, called LMC to see if they had a cross reference, checked with ebay sellers, performed various other online searches but no luck.
This one is similar to the yellowish ones you found but instead of a hole for the pop rivet, these have a prong that goes into the truck similar to the over-the-wheelwell trim.
Scott
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Camano' 1986 F250 Supercab XLT Lariat 460/C6
'Chanute' 1980 F350 C&C 400/NP 435 - Gin Pole
But there ain't nothin' wrong with the radio