B Dudley;
I stand corrected on the Parkers, thanks. I am much more knowledgeable on Lefevers & this is true as stated on them. I only have two which are not 12 ga though. One is a 16 with XX frame & it is narrower across the bar, the other is a 10 & it is wider.
As I recall the L C Smith Plans & Specifications book shows same dimensions for the bar on all sizes, with again the standing breech varying.
As s tated I do not have any Nitro Special but suspected this was the case with them as well.

Miller/TN
I Didn't Say Everything I Said, Yogi Berra