Was an outgrowth of Stevens' own R&D, I've always assumed, in response to the introduction of smokeless powder.  Grant does not speculate, nor anyone else I know of...except Ray Smith.  Someone mentioned his Pope book, leading me to look it over again for the first time in decades; he attributes it to Pope's redesign of the 44. When you read about all the other responsibilities Pope had while at Stevens, seems inconceivable he'd have had the time to involve himself in such a project, even if he'd wanted to.  Wouldn't be surprised if Pope told Smith such a story, but that alone is insufficient evidence.

Don't believe either Smith's claim of Pope's invention of the so-called Pope Rib, which he allegedly designed for his friend John Sidel.  They were indeed friends, but considering that Sidel gave full credit to the designs & ideas of four others named in his 1899 catalog, it's hard to believe he'd have failed to do the same for his friend Pope. Sidel gives a rather vague description of what I take to be a rib used only on his highest grade scopes, but calls it a "slide."

If I recall correctly, 38-55 was the 44's largest cartridge because of the action width.  To safely go above 38 caliber, a wider action was needed.  If Stevens was considering a larger caliber, they had to produce a wider action, 44 1/2.

Didn't the 44 frames vary?
For the No. 51, 52 and 54, Stevens said to add a 1/2 pound to the total weight of the rifle for the "large Schuettzen frame".
The 44 1/2 design came in two widths, with the narrower version being the 0 44 1/2 and 56.