The second version of Super-Takumar 50/1.4 with 7 elements optical design.
Lens can be identified from its predecessor by observing where the "IR" mark is. If it is inside "4", it is the original 8 element design if it is outside "4", it is the later 7 element design.
The later design was not modified other than SMC coatings up through the SMC Pentax 50mm f1.4.
The use of a rare earth element in the rear groups cause yellowing over time in the later versions. This yellowing can be bleached out by exposing the elements to UV in the form of sunlight or UV lamp for weeks.
One week under 11W UV tube removes about 70% of the coloration, one month roughly 95%.

For the full list of Takumar 50/1.4 versions and more info visit Asahi Optical Co. Super-Takumar 50mm f/1.4-16 (v1) page.