I'd hold off on naming a model number yet. I'd think it not a 6420.
It looks to me like a parts gun. The upper receiver isn't orginal, nor is the stock.
Serial number range is correct for a 6500, 6520, 6420, and probably an early 6600.
I'd guess that this rifle started out as a 6500. All the other configurations aren't worth busting up. That would also explain why the upper and stock was replaced, and the barrel was probably from another gun or source.
Either way, it still looks like a nice peice.