As far as I know , all planes consistently have the gun on the left side of the plane as the lower number key.
What plane are you speaking of?
HiTech
That may be true when you are facing forward, but when you are facing aft and actually using the guns on the Me410 the left hand gun is reached with the right hand number key. This is a little bit unique to the Me410 due to it having three keys controlling one gunner position.
When gunning in an Me410 you don't think in terms of going to the left waist gun, right waist gun or tail gun. You just go to the tail gun position and then select the appropriate gunsight based on where the enemy is.