Originally posted by boriscleto There were 2 different versions. The only difference was the rendering of the text on the front ring.
One version has "SMC PENTAX 1:2/28" while the other version has "smc PENTAX 1:2 28mm". [I.e., one version has "SMC" (all caps) and "1:2/28", while the other has "smc" (full-height lower-case letters) and "1:2 28mm".] I'm assuming that one version is likely older than the other.
Since M lenses often (always) show "smc" and "1:[max aperture] [focal length]mm", while many K lenses show "SMC" and "1:[max aperture]/[focal length]", I'm gonna go out on a limb and suggest that the "SMC" and "1:2/28" may be the older version, while the "smc" and "1:2 28mm" may be the newer one.
I have no idea if there are any other differences between the versions.
In any event, my specimen has the "SMC PENTAX 1:2/28" (older?) text.
EDIT: I suppose that it might be fruitful to look at serial numbers for the two versions to see which is older and which is newer (?).