This Sigma lens appears to be old enough to have had a C/Y (Contax/Yashica) mount option. Apparently there were a number of these made in the C/Y mount -
here. The C/Y mount is very close to the K mount. Zeiss helped Pentax back in 1974/5 design and develop the K mount. The partnership broke up for whatever reason and Zeiss next went to Yashica looking for a Japanese manufacturing partner for their Contax cameras where they developed the C/Y mount (hence the name). The C/Y mount is essentially the K mount with the flanges a tad thicker, so that it's not physically compatible with the K mount. Its close but no cigar. I am thinking that the flanges are a bit thicker, and hence the lens is getting stuck half way when mounting the lens.
I really do not know of any specific way to tell the difference between the two mounts - other than comparing the thickness of the flanges on the mount. There are no stampings that I know of for C/Y mounts - Pentax mounts from third party may have PK stamped on the mounting flange. I have only had to deal with the C/Y mount when it was on a Contax lens. Due to essentially having the same registration distance, there really is no adapter that can be used.Yachica lens with the C/Y mount so as to fit on to a Pentax body.
The forum had a member - Rio Rico who was famous for using his Dremmel Tool to shave the thickness of old C/Y mount lenses down, so as to fit on Pentax bodies. Note - You would NEVER want to do this to a Contax lens since it would damage its resale value.
Hope that helps...