The screws which hold the shaft is not in good shape. The screw head has been "screwed up" and thus they could not be removed by normal way. Being unable to remove that plate, the "smoothing" effort on the focuser draw tube was not completely done.
Today I used a drill to kill that screw altogether and I managed to remove it without any damage to the focuser. After that, I have replaced all four screws there with much better screws. I guess those original screws were killed by one of the previous owners in order to put on the JMI focusing motor. One of them was not original when it comes to my hand.
Now, they're all the same and I wish that I can DIY another focusing motor for it myself after my TG-SP II motorizing DEC project.