pictures are always the best to identify anything and more so when it comes to stencils. Pre 1930 horns had often split bell keys. This is what one of the best techs around here , Bruce Bailey had to say in a past thread about it
Placing the bell keys on the left is easier to make from a keywork standpoint, they can be one piece. Placing them on the right keeps them away from the leg if you play on the side. Placing the opposing on both sides makes it easier to have strength on the tubing as the tone holes don't sit so close together.