This Esterbrook ReNew replacement nib and feed unit marked 1551 is a Firm Medium nib made of stainless steel. Esterbrook ReNew nib units are interchangeable, and to change them, you simply unscrew one and screw in another one.

Please note that Esterbrook ReNew Point 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.