1975 duster fuse block hold down pic needed

The plastic piece that is circled in green is threaded. The thumbscrew is screwed into the plastic part.
If you were to hold the plastic part tightly, then you could unscrew it and have two pieces. The thumbscrew doesn't hold the threaded plastic part firmly against the metal bracket because when you rotate it 90 degrees both pieces must turn allowing it to drop through the slot.