It is one end of a set of tire pump clips. This one looks like the end that the valve connection went into. Most pumps nowadays are designed to fit between the seat post tube and the head set tube. To fit into the various size frames, modern pumps come in different sizes. In the old days, pumps came in one size and fit between clips. Some were brazed on the frame, others were fastened with clamps and screws. What you have is one of the later.