The "fuel filters" mentioned are firearm suppressors that are generally illegally used. Some times the filter is something that actually resembles a filter, sometimes it doesn't look at all like it would do much as a filter. Sometimes it's just an adapter that allows you to screw a real automotive oil filter onto the end of a gun. The other name for them is "solvent traps" which is some sort of cylindrical container thing you are supposed to attach to the end of the gun to collect a cleaning solvent that you pour through the barrel. No one really cleans a gun like this, you use the solvent on a swab attached to a metal cleaning rod. The internals just so happen to look like a typical suppressor but there is no exit hole for the bullet which you are supposed to drill yourself. Some even have a punch mark where the hole should to help align the drill. The illegal act is drilling that hole without filling out the ATF paperwork and paying the tax but even just possessing the "filter" or "solvent trap" might land you in hot water with the feds if they could make the case that you would have no use for such a device other than to make an unregistered suppressor.

The auto switches are drop-in mechanical pieces for a gun to convert it into burst fire or fully automatic, skirting laws, taxes, and registration. I don't remember the incognito name they use for these but there are very specific designs used for specific guns so once you see a picture, it's obvious what it actually is. Because these things are so specific in how they are built, there's pretty much no way you could escape conviction if you owned one and it was not registered. The feds could just say it looks like an auto switch and if you can't prove that it existed before the automatic weapons ban, then you are breaking the law.