[frank]

Thank you both, Popo and Markus. After checking the area more carefully, there is indeed a screw there inside the upper red square area in my image. So, they should be thread lock as pointed out by both of you. While inside the lower red square area, I did not see any screw (but the brownish material looks the same kind). Now I guess it could be some residual of the thread lock material when the lens was assembled.



Popo: I touched the material in the lower red square area with a teeth stick, it is not liquid. It is indeed hard and looks shiny.



Best regards,

Frank