Found this Tele-Astranar 400mm f6.3 lens in a thrift store for $15. The thread mount looked like a M42, and although the diameter was about right, the threads on the Astranar looked dense (finer) than the M42 mount adapter I have for my NX1.
Then after doing some research I found that threads where for a T-mount (or T2-mount) so I ordered T2-to-NX adapter online.
The Astranar is a 'preset' lens meaning There is a second ring which lets you control where the other aperture ring stops (the one that actually closes the aperture down). So if you recompose an image you can quickly turn the aperture all the way open so the viewfinder is bright, then stop back down to your shooting stop without taking your eye away from the viewfinder (below).
Below are test photographs. All images are at f-stop 6.3 (open). All images below are 'as-photographed'