Silencers are their own device and need the tax stamp separate.
Speaking only of your AR pistols, as there are many other guns. You can SBR both pistols, then you can buy one DIAS or Link that was registered as a machinegun properly before 1986 and use this conversion device to move back and forth into either of your guns. If you do not have the separate SBR registration then the conversion device must stay in that one only host weapon as once you remove it you have created an unregistered SBR.
If you intend to convert only one firearm with the conversion device and are happy to leave it in that one gun then no SBR registration is required.
Before you get all giggle excited, a registered conversion device is going to cost about half as much as a new car.