I have the shutter lock correctly set according to the manual at the fully released position BUT with film in the camera (either 120 or 220 back), dark slide out, shutter cocked I cannot depress fully the shutter button to fire the camera.
You can not depress fully or it just does not fire?

I tested on mine and it does not depress fully only if shutter lock is on. With dark slide in and shutter lock off it depresses fully but does not fire. I use newer backs so it may be different on yours.