This doesn't directly answer your question,but the K24/3.5 is quite simply a Takumar 24/3.5 with a bayonet mount to replace the m42 mount. The K24/2.8 is a different optical design. Perhaps the 3.5 is simply an older, less efficient design made before the advent of SMC to allow smaller lenses.
K24/3.5 from Boz
K24/2.8 from Boz
My personal opinion is both are wonderful lenses, but each has a unique character. I think the 3.5 maybe a bit sharper but less contrasty and the colors aren't as punchy. The 2.8 produces a more robust image with higher contrast - and that maybe hides a tine bit of sharpness. Some of that may be attributed to sample variation and/or changes to coating. They're both excellent lenses.
[EDIT:] In the Forum reviews Phil (gofour3) speculates that the 24/2.8 is built and feels closer to an M than a K and was released at a time concurrent with the M lenses, replacing the K24/3.5. He supposes that had Pentax been able to design this lens with the 49mm filter ring they would have labeled an M-series lens.