Originally Posted by d.bradley
I guess this is a seriously rare lens!
Yes, it is. All I can tell you is, the Super-Multi-Coated Takumar version appears to be the same as the SMC Pentax ("K") version, with 6 aperture blades. The Pentax-M has a different optical formula and 8 aperture blades. The Pentax-M formula looks a lot like the formula for the Pentax-M 135mm f3.5 - I don't know whether that means similar performance.