The correct mag for a Sht.22MkIV* is a hollow shell with 22 stamped into the side. Sometimes they weren't stammped 22 though. The CNo.7 trainer does have the feed ramp you are referring to, but it's based on a No.4 action, not the same setup as your rifle.

As for the screw... no ideas there. Maybe ask your gunsmith to make you one? If so, provide him GOOD pics to copy.