Pentax is noted for several "classic" lenses and this is one of them.

It is one of my favorite landscape lenses. When looking for one
in the used market be aware that there is another Pentax 135mm f2.5
which was a low cost lens made in the 1980s.

How to differeniate?

The classic is a 6 element in 6 group lens with a filter diameter of 58mm.
Originally designed in 1968 it came out as a Super Takumar (two coatings only),
then as a Super Multi Coated Takumar (seven coatings), then as a Pentax SMC K mount
(with rubberized focus ring). All of them have the name on the inside front of the lens
and they all share the same optics.
They sell occasionally on ebay for $2-300 USD.

The low cost version is a four element in 4 groups with a filter diameter of 52mm.
It was produced from 1980 to 1988 by another company for Pentax. It has the name
TAKUMAR (BAYONET) 1:2.5 135mm on the outside focus ring. This is why it is sometimes
referred to as a Takumar 135mm f2.5, which leads to a lot of confusion. It does not have
the super multi coating, has a rubberized focus ring and attached sliding shade, and is
covered in Bojidar's site
http://www.bdimitrov.de/k..._non-SMC/tak_135f2.5.html
under non SMC Primes. When you see poor reviews of a Pentax 135mm f2.5
such as on Stan's Pentax Photography site,
http://stans-photography.info/
this is the one they are talking about. It is commonly on ebay for $40 -$60.

Stan's site refers to the lens as a poor man's A*135mm f1.8
as the optics are almost as good but where the A* lens goes for over $1000.

To keep it short, I will only mention that Pentax has also made a f2.8 and f3.5 version
in various flavors and optical designs over a long time.

Enjoy.