That looks like a square boss for a set of top-mount shifters; perhaps whoever equipped the bike only had a set of old-school band-clamp shifters on hand at the time. The remaining braze-ons give this bike such a "lean/clean" look otherwise, the band clamp is a bit of a sore thumb. Installing the right top-mount set for that boss would definitely do the overall appearance justice.

Wonder what potential ID information lurks on the steerer tube.