I have the exact same scope mount on my MS. The stud you are trying without success to push in IS made to push in, so yours must be seized. Once pushed in ( rearwards) the claws on the rear scope mount should release and the scope may be tipped up in an arc forwards and the scope removed. Try Kroil, it's the best product I have found to creep into rusty seized parts and release them.
Thanks LW! I will hit it with penetrating oil and then heat.
The fixed screw that the stud screws onto looks off centre in the hole in the mount base. It may be affecting the operation of the release button.




















































