The problem !
 * "KR" "P/K-A    R-P/K" lenses

Lenses made by a few companies before Auto Focus came out (Vivitar / Sears and others) did not want to make two different lenses for the newer auto / program bodies in late 70s / 80s.  These are bodies that could determine what type of lens was on the camera and the maximum aperture of the lens.  So you could place the lens on the camera and put it in Auto/Program mode and just shoot.  That was all fine until Pentax camera came out with an Auto-Focus body and lenses as well as the digital Pentax bodies.

To make a lens that would fit the auto/program of a Pentax and Ricoh they just needed a pin on the lens that would contact a pin on the body.  Ricoh bodies/lenses had to be different and put the pin on the opposite bottom side of a Pentax body.  That was fine until Pentax wanted to make Auto-focus lenses and bodies.  Pentax made the multiple contacts to work the auto-focus on the opposite side of their "Auto" pin was...  right where Ricoh made a pin (pin and spring behind it) to work their "auto" bodies.   So if you take these 80's lenses made by Sears, Vivitar that are marked R-PK (meaning Ricoh and Pentax auto) the extra pin for the Ricoh can fall into one of the Pentax AF contacts and lock in.  

There is hope, it's a struggle, but will get the lens off.   Go to the site below for information.  If you don't know what a feeler gauge is in the instructions, they are for measuring distance of rocker arms in cars. So a "car guy" should have these ultra thin metal blades to slide under the lens in the area of this stuck pin.  The pin tip is actually rounded, so a feeler gauge should slide under the pin, and then rotate off the lens.  Some people say they have no problem, others worked hard to get the lens off.

Here is Photo.net users that tried a few different ways.
One had to restore to a very, very thin saw blade.
This is the image of the flat Ricoh pin before removal from Photo.net.

The link below shows how to remove that pin for good.  I have a few Sears lenses that did have both pins.  One is a Sears 135mm macro.  It was made by Ricoh (that now owns Pentax) and is a pretty good lens.  I would suggest buying that special screw drive, as suggested in the article below, the Phillips screw made by Japanese have a much deeper angle of cut.  I found out when I tried to disassemble Chromebooks at work.  I brought in my "JIS" screwdrive from home and those tight screw came out very easy as the tip fit perfect and the larger handle gave better grip.

These are only manual focus lenses with the "A" lock on the aperture ring, made by Sears / Vivitar and a few other 3rd party lens companies.