What you have is actually half of a cylinder head from a Pontiac inline 
six-cylinder from the 1920s-30s.  They were cast in two pieces, one half 
for the front three cylinders, and one half for the rear three.  I think 
you have the rear, based on the position of the water outlet and circular 
notch at one end that goes around the distributor.  You can see a picture 
of the complete motor at the webpage below:

http://www.classiccarsforsale.co.uk/classic-car-page.php?carno=6158

I would suggest contacting the Towe Auto Museum in Sacramento.  They have 
an auto collection composed of many makes of cars and trucks and people 
who know them inside and out.  They should be able to narrow down the date 
of production based on the part numbers embossed on the head.