At one end, it will fit most Pentax M42 screw mount lenses and double
their effective focal length.  At the other end, it will mount to any
Pentax screw mount camera such as a Spotmatic.

If used with a Pentax screw mount to K bayonet adapter, it will enable
you to use M42 screw mount lenses on most Pentax cameras with M, KA
and KAF mounts.  Some of the later Pentax film cameras with autofocus
(plus the MZ-M) need lenses with an electronic linkage, and this
teleconverter doesn't provide that.  But it will work with any Pentax
Digital SLR.

It's for the M42 screw-mount Pentaxes: Spotmatic series, for example.