The way you write it, its so clear cut. If its that clear, why has it taken them years to enforce the law? Who should get fired for allowing all these illegal suppressor to be sold?
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I have no doubt that internally the ATF always looked at the 'solvent traps' as a premade silencer. In a different time and under different leadership , they decided not to press the issue and allowed their sale under the 'if it has any other legitimate purpose' interpretation.
Just because the cop lets 10 cars going 15 over pass before he finishes his donut and pulls you over the speeding law isn't suddenly invalid.