The 908's, 910's (etc) were the budget line of Smith pistols when they came out. Fit and finish isn't as high a priority on them, and they should have a few plastic parts (like the guide rod) that the other models don't.
As alluded to above, the aftermarket has mostly given up on these older pistols and shifted their attention to the current models. At this point, it's even hard to find OEM parts as S&W threw in the towel and pretty much abandoned us.