Found it in the dustbin of history (1963 parts price book). 375790 was superseded by 277581. Then 277581 was superseded by
“Use (1) 378580 Propeller, (1) 304575 Drive Pin, (1) 304576 Thrust washer, and (1) 376731 Prop Nut.
All this was because the original rubber cushion hub and brass SHEAR pin was replaced by a rubber “Slip Clutch” hub with a stainless steel DRIVE pin. Competition y’know from the folks that advertised “No Shear pin”.
steveD’s chart above shows 378580 as being 10-3/8 x 12-1/2.