There is also the Drop In Auto Sear, DIAS. A little piece of metal that’s Transferable (Pre ‘86) for the HK platform is now close to $45-50k. One for a AR platform is $35-40k. A Lightning Link is about $23k.
There are states that currently forbid ownership of FRT or Binary triggers. A binary trigger functions when you pull the trigger, you get a bang, then when you release the trigger, you get another bang. A quick double tap if you will. With practice, you can get pretty fast with it.
As to letter rulings, at all the Alphabet Agencies, if the ruling isn’t written into the law, they can no longer arbitrarily give their definition of what they think is the law.