It’s not. It’s illegal to not have a tax stamp for it and it’s double illegal for a felon to be in possession of one.
L
Glock auto sear switches are all POST 86 so none of them are legal to have a tax stamp for unless you are a class 7 manufacturing FFL and are making dealer or LEO “samples” and then you can only transfer that post 86 weapon to a class 3 ffl who is also holding dealer samples for LEO or military sales in short none of the glock auto switchs are legal for a civilian to own anywhere at anytime.