Here is my FirstSix made by what would become Petri. I have taken the focusing ring off and the screws were buggered, I use a little liquid wench and a very small screwdriver. With the focus ring off you can unscrew the front element (marking infinity position first) and then clean the thread throughly. Then reassemble with the very thinnest oil you can find. Replace the focus ring at the infinity setting and either do as I did live with the buggered screws or re tap.