That is a good point, and it seems poorly drafted, but IIRC, they called it the “plastic gun” ban at the time after the misconception came about that Glocks were undetectable. There weren’t plastic guns then and aren’t now.
My WAG is that when drafting the bill, they told the lawyer drafting it that they wanted to ban plastic guns and also ban plastic major components of guns. Keeping up with the WAG, imagine in a world with a two man terrorist team wanting to get a gun on the plane. The lower serialized receiver or frame is made of plastic and is undetectable and the rest is metal.
One guy can put the metal stuff in his carry on and tell the gate minder (not TSA at the time) that he is simply transporting gun parts to sell at his destination. The other guy can carry his plastic frame/receiver through undetected. Then they get on the plane, go to the lavoratory and reassemble a functioning firearm.
I guess when you write a law outlawing magic pixie dust, it really doesn’t make much sense.
ETA: Re-reading the law, it doesn’t say it has to look like a traditional component. It says that when placed under an x ray, it doesn’t “generate an image that accurately depicts the shape of the component” “The component” not “how a regular gun component looks.” So, on my read, I see that as, hold toaster gun up beside the x-ray pic. If the two images are the same, then good. If the images are different, then bad.
But I am unsure how the BATFE reads it.