I have been using the target adjustment knob for elevation for a few years and I agree I wish they had a quick release like the Lyman. The locking screw is a part that you can order from Williams. It is called the WMS Marlin 6-48 Giblock Locking knob, they cost about 8 bucks each.
When you get them they are a pinch too long. I took off the sight and screwed it in all the way. Then I used a 1" wide sander and took off the extra length. I like it being tight now instead of loose.