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Check what mount your camera has - as well_is_it said there's the m42 screw type, after that pentax had it's own "k" mount.
Always found this database pretty useful: http://www.pentaxforums.com/lensreviews/ (scroll down to "Film era Pentax K mount lenses", then check the "A prime lenses" and "M prime lenses").
All these are of great built quality, really a joy to use!
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the lenses I've got are threaded, presumably that means M42 type?
EDIT: just re read @Well_is_it reply.
oooo, what should I be looking for lens wise on the pentax?