Unlike K mount adapters, this m42 adapters are made in metal construction which are better built for longer lens that has heavier weight. I actually prefer the use of this 2x TC when I am using my 200mm as the Kiron 2x TC has a better screw thread in mounting to my K body than my my 200mm lens alone.
Kiron made 2x TC is quite decent
Kiron 2x matched multiplier
next to Pentax-F 1.7x AF TC