This Venus replacement nib and feed unit marked Medium is a Firm Medium nib made of stainless steel. These are interchangeable with Esterbrook nib units and to change them, you simply unscrew one and screw in another one. These will fit into vintage Esterbrook and some Osmiroid pens.

Please note that Venus nib units are designed to fit vintage Esterbrook pens. These will fit into the modern Esterbrook pens that can accept the MV (Modern to Vintage) Nib adapter, but only by using that adapter.