I think that's clearance, not engagement. Check to see if it engages something else? I've run across something like this once in a Chinese lathe: gear that engages nothing. I figured it was part of the casting that they copied form another machine, but had no purpose on that machine. The lathe wasn't mine, and it worked fine when we reassembled it, so we ignored the extra gear. If I remember it was in the Rack and Pinion gear train, but I can't remember where. (Sorry I just don't have a photographic memory, and this was 20 years before digital photography)...