It is a small lens, intruduced in 1975, to replace the M42 version, adding Pentax SMC coating and a minimal hood to hold a 65mm metal slip-on cap. It has a filter turret with Y48(Y2), Skylight, none, L39(UV), O56(O2).
There are two versions, seems only the capital letter of the SMC in the name is changing:
1975 SMC PENTAX FISH-EYE 1:4/17 cat.number 21240
1977 smc PENTAX FISH-EYE 1:4/17 cat.number 21240
The slip-on cap is the 58mm one, the same one you can find on the first version of 28mm 3.5 M42 and on other Pentax lenses of the same age.
Digital use:
First of all, any Pentax or Samsung-Pentax mount D-SLR can use this lens, as the M42 previous version, with the M42 adapter. On Canon EOS cameras, with an adapter you can find easily on eBay, you can mount the lens… but a caveat! Pentax K aperture pin may protrude inside the camera and should be removed… could prevent you from mounting the lens, as well could be in the clearance of the mirror path if you use a full-frame camera. No problem with any mirrorless.
from left to right the 3 lens variations, from the early screw mount to the Pentax K version. 
from lens to right the mount part of the lens, from early M42 to PK.
