Finally got around to looking into the casting numbers on the Y block in my Model A.  The block says "B9AE-6015-F" and the heads are ECZ-G, but I have no idea how to tell if they're big or little.  It's cast into that spot next to the driver's side intake coolant mating surface.  
One source doesn't even have those as options.  One suggested it may be a Mercury block.  
What do the experts say?