One of my K31's came with the front tunnel sight, but without the rear diopter.
The front tunnel sight has a post insert that actually worked well with the standard rear sight, except...it is so high that I have to set the rear sight at 800 meters to hit the bull @ 100 yards.
I fired a couple rounds of GP11 at the target (a 12" target) at 100 yards...nothing. I finally figured out that it must be low...because the front sight was so high...I set the rear sight at 300 meters and got the two at the bottom of the target backer. Cranked the sights up to 500 meters and got the one halfway up. Cranked the sights up to 800 and finally hit the bull, while aiming at the lower red target dot. I've since gotten a Swiss Products diopter sight for it.