This Is A Short Propeller Adapter Nut from Dave Brown Products.

Plain black, CNC machined steel construction.
Use with 2-1/2" spinners and under.
This prop adapter kit tightens the backplate and propeller onto the
engine, then using a single bolt-on design, holds and centers
each spinner perfectly on its baseplate.
Will fit the following engines:
OS Max .25-.60, SuperTigre .25-.60, Fox .19-.50,
Enya .25-.53, K&B .19-.65, OPS .40-.60, Webra .28-.80