The hex bolt on the rear of RS520 is taptite 3mmx10mm (10mm is the length of the bolt tab part)
It is not a typical bolt used where there is a tab.
This is a bolt that is forced into a place where there is only a hole and a tap is inserted.
It is also called a double bolt.
May I know the screw size / specification of RS520 on the back of the machine (black in colour).