" I'm no expert (hell, I'm not even a novice) on polymers or how one would go about producing such a lower in that fashion, but I can see how someone might think they made the lower, then filled in the area with a different color as a means to produce this product."
Very interesting thought as the "lower" has already been produced. A fully completed one. Basically produced a lower, filled it, then claim its an 80% lower.
With that in mind any lower could be filled with wax, polymer, etc and be called an 80%. So could a company such as spikes make an aluminum lower, fill it with polymer and call it an 80%. Customer wood only have to remove the polymer. Just as the above lowers.
Just some thoughts. I have no clue about 80% rules and regs

