The hole on top of the gas tube is for an anti rotation pin when using a JL Billet handguard. Pay close attention to this upper, it has flared areas behind the barrel nut threads and is not mil-spec. Only select handguards including the JL Billet rails will work without modifying the upper. I'm speaking from personal experience, I own several JL Billet lowers and uppers.
They will match up, but the cosmetics are really intended to go with a JL SKLTR lower receiver.
The lower receiver pictured is a JL Billet lower receiver. Unfortunately, we do not carry that specific lower.
Best compatibility would be with the JL billet handguards: https://www.opticsplanet.com/jl-billet-handguards-and-forends.html
Yes, it should be removable.
Yes, you can use a standard delta ring.
No, this product does not require an FFL. This is only for the upper. They are just showing it in the pictures with the lower, so the customers can see what it would look like together.
Yes the upper is fully compatible with dust covers but is a "slick side" upper and does not have a Forward Assist. I mentioned this in the other answer I left but pay close attention to the upper as it has flared areas directly behind the barrel nut threading and is not mil-spec meaning only select handguards will fit this upper without modifying the upper or handguard.