Hi,
Just adjust the screw for the shell stop (the screw nearest the barrel) to just snug and that should get rid of the slop. I have built two 520s and needed to tighten both all the way in. The bushing/nut screw should definitely be tightened-in or the bushing may drift out and lock the action. The problem is you need a special tool or fabricate something to hold the bushing in as you tighten the screw from the outside.
As far as your lifter spring goes, it depends on the year. Since one of my double-hump 520s is a 959--A model (1925 1/2 - 1928), like yours, it should use the spring that looks like a wishbone, or roughly a "V"-shape. If yours is different, check with Numrich Gun Parts Corp for a replacement spring.
Hope this helps.
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