The 31mm 1.8 is a full frame lens, older design, but still a very nice lens. I have the 77mm, which is also excellent. Both the 77mm and 31mm are the limited lenses, and FA. Full frame.
Other wides to consider:
35mm Sigma Art, much heavier but IMO a bit better than the 31mm Pentax and a bit cheaper.
14mm Rokinon, hard to beat this lens, very inexpensive and still is the champ of all 14mm F 2.8 lenses, (nikon, canon, Sony etc). No AF, however.
15-30 Pentax, beast, but again, an excellent optic, I would rate as good as the Nikon 14-24 and Canon 16-35mmII lenses, both of which I have used. Pentax 15-30 is much heavier than anything else, has a curved front element and is not filter friendly. Also mine is not tack from 24mm to 30mm, but the area I really use 15-21mm is excellent.
Samyang 20mm 1.8, still not shipping but still should be a great lens if it follows the footsteps of the 14mm. Remember Rokinon, Samyang and Bower are all the same.
Older Pentax film lenses, mainly the 28mm that are found on ebay can be good depending on lens and condition.
Paul C