Hartzell Propeller Spinner C-1904-3 Click here to purchase or contact the seller of this item on eBay
Current Price: 599.00 USD

As shown, as is: a Hartzell Two Blade Propeller Spinner Dome.

I believe this is a Hartzell Propeller Spinner Dome P/N C1904–3. 

The diameter is 12 inches at the base, as shown. 

Please note a small deformation shown in the last photo. 

Be certain this fits your application. 

Used on many Cessna 170 172 and Piper PA-28 aircraft.