There are several versions, but without optical difference, which is good because the 7-element 50/1.4 Takumar design is a legitimate claimant to the title of sharpest normal lens ever made; and bad for another reason, noted below.

There was a cosmetic change from scalloped knurled focus grip on the "Super-Multi-Coated" version, to the later "SMC" version with rubberized focusing grip which was cheaper to make.
It was simultaneous with the name change.
Some users have also noted that the "feel" of the focusing on the earlier, Super-Multi-Coated version is silkier and smoother, both when new and as the lenses age.
I can't remember a reason why this should be so, but I haven't taken one of the SMC version apart recently, either.

The bad news: unfortunately, both versions, and the SMC Pentax 50/1.4 with 52mm filter ring, have an internal element made of material known as "hot glass" which turns yellowish to brownish with age.
The affected lens element contains thorium, a radioactive rare earth element. The source of brownish tint isn't the thoriated glass itself, but Canada balsam (optical glue), which turns yellow or brown owing to radioactivity of the glass.
Several lens makers including Pentax, Kodak, Leica, and Canon used this manufacturing technique in the 1940s to early 1960s to lower the index of refraction of specific lens elements.
The yellowing is curable by exposure to ultraviolet or sunlight, as has been discussed on the Spotmatic Yahoo! Group.
The earlier 8-element 50/1.4 Super-Takumar and the various 55/1.8, 55/2.0 and 55/1.9 Takumars do not turn yellow, nor do the newer 50/1.4 SMC Pentax-M/A/F/FA.