OM came to the same conclusion I did. 12 threads per inch is a pitch of 0.0833" - very close to the 0.0853 shown under the 4 on my lathe plate. Use the half-nuts, you get 4. Use the feed selector and the clutch, you get 12-ish.
The clutch doesn't let you engage the feed at the exact same point you did last time. Plus, it can slip - actually a good thing under some circumstances. It would be virtually impossible to cut a clean thread, even a 12/inch without using the half-nuts.