I understand the confusion, since the pointed, rounded profile of Sheaffer writing instruments is usually associated with the Balance line, but I really do not think this mechanical pencil belongs to that period. They were usually made in Radite (green nitrate celluloid) and shaped differently.
The owner would sign a card, and a skilled engraver would faithfully cut their signature into the band. It was quite an expensive option and, as far as I know, generally reserved for the more luxurious sets.
It is difficult to pin down the exact model from a single photo, but at first glance I would be inclined to say it is more likely the pencil from a Signature TM (Thin Model Touchdown) set, from around 1950 or so.
The reference for Sheaffer fountain pen collectors is PenHero (scroll down to the Sheaffer section). This is where I would start a search, especially comparing the pencil with advertisements to see which models match.
