This is an unusual variation of a hump filler. The pen is stamped “Welty FOUNTAIN PEN WATERLOO IA.” There is no patent notation, pending or issued. The pen came to me with no nib and a mismatched cap. It is clearly a relative of the Welty/Evans hump filler. This Welty hump filler utilizes a middle joint in the barrel to act as the lock for the hump.
