Vestigial frame tube or supernumerary top tube.
Most bicycle scientists feel the answer is inconclusive.
Either way, go ahead and use it to support a frame pump.
That's how most of us use it.
Whatever you do, don't finger it obsessively while riding.
__________________
WTB: Slingshot bicycle promotional documents (catalog, pamphlets, etc).
WTB: American Cycling May, Jul, Aug, Oct, 1966.
WTB: Bicycle Guide issues 1984 (any); Dec 1985; Apr 1994; May, Jun 1996; May 1997.
WTB: bicyclist May, Dec 1997: Jun-Dec 1998.
WTB: Bike World issue Jun 1974.
WTB: Litespeed head badge (circa 2000)