On POTN there have been a few 120-300OS failures dealing with the OS and AF systems so that's likely where the upgrades are plus the focus limiter switch.I'm a bit of a noob...why would you want a focus limiter at all? What does that do for you?
Say you're shooting something far away. But there's a fence or trees close (like you're shooting a bird or something). The subject keeps moving, so you have to keep re-aiming the lens. If you point at the nearer fence/trees, you will focus on that. But with the limiter switch, it will stay focussed far away, so keep focussing on the bird.
Or you're shooting at a track-and-field. Your subject is close, there's a nice stadium in the background. When the subject runs, you have to keep tracking, if you miss it for a second and point at the stadium in the background, the lens focusses far away, and you have to try to focus back on the subject (during which time Murphy will make sure you've missed your shot). Limit switch set to close means you always track your subject.