i have the similar gun which I believe to be a 'Royal'. Mine is choked extra full and full. I also have another 12 ga,model unknown and a 20 ga 'Royal' both are choked very tight. I do not have the right gauges at this time and rely on measurements somewhat as you do. My conclusion from the 3 I own is that they are choked much tighter than other UK of US guns I own. I am not sure if this is just the way Sauer chokes their guns.

Nothing is foolproof to a sufficiently talented fool.