Bill;
An old Lyman Handbook of mine dated 1951 gives 24 grs Red Dot as 3 DE with from 1 to 1oz shot & 26grs for a 3DE load with 1 1/8oz shot, all in 12ga. 24grs & 1 1/8 plus 13% is extremely close to the 27gr- 1oz load in the 10ga. A late 50's Lyman handbook had dropped the heaviest load of RD in the 12ga to 23grs-1 1/8oz for 3DE.
Not positive but I think they received a lot of their shotgun data at that point in time directly from the powder makers.

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