London Steel barrels were put on the very early No. 0 Grade and after 1912 became the Ideal Grade. The Nitro Steel barrels were put on a few grades and commonly seen on the Specialty Grade. Pre 1912 guns with Nitro Steel were the No.2 and No.3
I don't think it was a marketing ploy as these barrel cost more on each grade.
As far as the ads for post 1939 guns, the improved cocking I am not sure about, they used the same type of hardware for cocking. As to the strengthened stock, they did use a metal tube that went from the top tang screw (under the top lever) to the trigger plate housing. The screw went through this tube. This was to stop someone from overtightening the top tang screw. They also used a tube that went from side plate to side plate, this was also to stop someone from over tightening the side plate connecting screw.
