The article I have from Jack Lott about his H&H back action Jones underlever refers to the 577 3" version.
It's in the 1985 Gun Digest and was reprinted in "Gun Digest's Classic Sporting Rifles" 2012 edited by Terry Wieland.
The article is titled "Make mine a sidehammer rifle".
Classic Sporting Rifles has a whole section on double rifles with another article from Jack
"The double rifle it's care and feeding" which has a boatload of load data (Gun Digest 1979).
Also, from 1967, Elmer Keith's "The deadly dependable double" and several other articles.
There is also another whole section on single shots.
If you can find a copy of this collection it's well worthwhile.

Unfortunately I do not have a scanner so hopefully someone else can post some of this material.