Here is a link to patent drawings that should help.
Patent 953291 Item #25 sets the headspace by how far it's screwed on to the breeck block #9. To turn it you have to have the barrel off and then replace the barrel to check it - then remove the barrel to adjust again and keep doing this until the head space is set. This sets the distance for the locking bar # 27 - notice how small it is, this is all that holds the action closed so standard velocity or even CB's would be suggested. Make sure the locking bar is not bent or cracked. There are 2 version of this that I have seen, an early one that is fully machined and a latter version that is stamped from thicker material and then has minimal machining on the locking end. NOTE: the action will appear to lock up even with the locking lug missing but it is only being held by the action interlock bar that is released as the hammer falls.
The action inter-lock is tricky to install.There is a plugged access hole in the lower front right of the receiver that is only used for assembly - this is used to make sure that its end #34 (part also labeled with #30, 31, 32 & 33) is below the end of the operating rod #24 (also labeled #22 & 23). If this bar is assembled above the operating rod it will get bent or broken when either the hammer is cocked or the action rod pumped.
I would strongly suggest you get another Visable Loader that is know to work to compare with or have it checked by someone familier with them. You should note the inventors name, E. E. Redfield, he was the brother of John H. Redfield who founded Western Sights about 1909 which became Redfield Sights in the early 20's. E. E. Redfield also designed the Stevens 425 Hi Power (the Blue book incorrectly gives credit to his brother John).
As for "Misarable Loader" I think this is a modern name as you find many of these gun is very used and worn condition so they had to work reasonably well for a long time. Also they were made from about 1907 until 1932. Savage bought Stevens about 1920 and continued the production into the depression and 3 years after the introduction of the model 29. They even upgraded the design in the last years to a pistol grip and a larger fore-arm.
