DrummerBoy,
"What all this has to do with is that everything I read seems to indicate that, given modern steel, the toggle action of 73/76 doesn’t really give up all that much to subsequent developments. IMHO" Now that I understand your overall premise, I will say --- To a certain point, I will definitely agree with your statement about the newly manufactured guns. Just look over at the loading results of other shooter, in the other caliber reloading Stickies. Some velocities are impressive.
But you have to understand, that I believe and other also, that the 1886 was a 'Stronger' overall design, in a shorter receiver frame than its BP toggle-link brothers. Also remember that the '86 design with better steel, became the platform, for the revered Win '71 in 348 caliber. Even the 1892 design had special High Velocity smokeless loads develop for it in the early 1900's. Neither the 1873 or 1876 were graduated to higher velocity smokeless cartridges, until our modern times. But again like you state -- the newer steel does allow for higher velocities and newer cartridges like the 44Mag in an 1873 design. Sorry, I do not know about the smokeless loads for 45-70 Trapdoors.
But, I am now going to have to beg-out of this conversation because I only have the 49th Edition and it does not have what you are talking about. So, I cannot read and maybe re-interpret what you are reading. You see the 49th Edition was the first time Lyman even addressed the NEW '1876 replica' rifles and their reloading(my reason for buying it). All the information was in one comprehensive chapter, only 4 pages total. It only covered 40-60, 45-60 & 45-75. Not the 50-95.
Also this Edition only has 2 other 40's (besides the 40-60). Only one of them is what you are talking about, the 40-65. In its intro page it does state "This data is intended for use in modern, newly manufactured guns rated for smokeless powder. This data is not for use in antique guns originally built for black powder."
So hopefully others will be able to help you work through the newer Lyman Edition's intended information...