M42 adapter which?

The original Pentax adapter costs $ 21.95 at Pentax' own web store:
http://www.pentaxwebstore.com/product/9384

I, personally do not risk with Camera mount and lens mount. The price is not cheap but absolutely worth.

I was lucky myself: I bought a couple of the original ones at Ebay many years ago when I even did not have a Pentax K body. I had only some M42 lenses which I used on manual Chinon bodies. I paid $ 5 for two and when they arrived I saw that they were not used even. But that time is unfortunately over now.

regards,
Moji