It's for a shoulder carrying strap. The other mount is above the bottle bosses on the seat tube. One bicycle that used this feature was the Bianchi Volpe, which wass introduced as an "adventure" bicycle in the late 1980s. It was envisioned that the bicycle would be carried in some off-road situations, akin to a cyclo-cross bicycle. A circa 1990 Volpe would be my leading candiate. The serial number will tell us if it from one of Bianchi's Asian sources and the date of manufacture.