My opinion is so the recticle is fully visible with a straight-on view.
Given the most people are right on dominant the right-side offset would make sense.
You can find P-51 blueprints here North American P-51 B C D H K Mustang - Blueprints, Manuals,Data [fp_p-51] - $119.00 : PLANS.AERO™. I might suggest Google. I found this Yak-9 drawing, http://aviadejavu.ru/Images6/MM/MM-15/0451-05-2-12.jpg