Here's some info on Butler:BUTLER (W.I.) P.I.26690 Tree originated 1904 at Miami, FL. Fruit season Aug., Sept.; color, light green; weight, 16-24 ozs.; shape, obovoid; skin, smooth. Seed, medium to large. (CAS Yearbook 1950) Origin, Florida; race, WI; Flowering group A (Lahav & Gazit)(A USDA selection in Florida; fruited in 1909, propagated from 1914 to 1918) pear-shaped; medium-large; skin smooth; seed of medium size, tight in the cavity. Season: Aug.-Sept. No longer grown in Florida. Cultivated in Puerto Rico.(J.Morton)
Hass is pretty well known. Should be an interesting cross.