Spitfire Gunsight adjustment?

[SEE]

I adjust the top gunsight ring for my preferred guns convergence at 200yds but not sure of the function/purpose of the lower ring as it seems to do exactly the same as the top ring regards placement of the cross hairs with respect to the edge of the gunsight ring. The upper ring is calibrated in Yards and the lower ring in feet but covers a much smaller distance - I am curious as to its function/purpose and cannot find any info?