FWIW, and IMHO, a spirit level has some value in target shooting, especially at long range, but you need to be well stabilized to keep the bubble from rocking side to side, and if Im watching a bubble Im not watching the wind flags, mirage, target or sights.
They do look 'purposeful' though.
IIRC, my list has four 17A sizes, maybe they are not all available now.