From Ammoland:

A solvent trap is designed to catch solvent from a firearm when the gun is being cleaned. The solvent trap attaches to the front of the gun by using a threaded barrel. The solvent collects at the bottom of the tube of the trap. Solvent traps have been popular amongst firearms enthusiasts that with possession of proper Licenses can convert the traps into fully functioning suppressors by drilling out internal plugs and the closed tubing end.

“Solvent Traps” or “Fuel Filter Kits” are not regulated by the Gun Control Act (GCA) or the National Firearms Act (NFA), because and let us be honest here, we are talking about metal tubing. But that might be changing with the new rules expected this month. Certain senior members of the ATF leadership see solvent traps as a loophole to get around NFA restrictions on hearing protection devices, aka firearms silencers.