Originally Posted by raymeedc 
M series 135s are commonly available on Ebay for about $50. I picked up a mint condition example recently on Ebay for $46. No need to settle for an off brand.
Since when is the purchase of an "off-brand", "settling"? My main consideration for a Pentax-M 135/3.5 over the other alternatives would be size, weight, and the SMC coating. Performance compared to a Tamron, Rikenon, or Vivitar of similar vintage would be equivalent with a bit of a nod given to the generally faster maximum apertures on those "off-brand" options.
At least that was my train of thought in 1982 when I was considering my K-mount options.
Steve