I should, but don't know the entire/correct answer to this. I can tell you this much:
Any/all parts fit to the frame are numbered. So too are any/all parts fit to parts fit to the frame. The reason being S&W finished guns in pieces---the various pieces placed on racks/fixtures in the ovens/tanks (depending)---and when that's done, all these (numbered) pieces are sorted, and find their way back to where they belong. I say they're sorted, I'm inclined to believe they were initially placed for finishing in order in the first place, rather than sorted after the fact----makes more sense.
My knee-jerk reaction to the fact some extractors are numbered, and some are not, is that those not numbered are replacements---and any that have been in Japan (or anywhere else where they were subject to an organization's armorers) may very well be numbered to a gun other than that they're on now. Jinks tells me S&W's service/repair folks work on one gun at a time---with no parts from other guns on their bench so, 1. There's no chance of a mix-up, and 2. There's no need to number replacement parts.
Ralph Tremaine