If you look at enough German proof marks, you will notice many that gave two different "gauges" shown by the marks. Examples include 12 and 13 or 20 and 19. The one in a circle is the size of the chamber and the other is the diameter of the barrel ahead of the chamber. Admittedly without checking, I think it is possible that a .410 bore chamber is around 36 ga. while the bore diameter, ahead of the chamber is around 67 ga. Seems reasonable to me- what say-you?
Mike