The only way to tell is by the date code. The casting number for the housing is like the end cap. It was used for a million years on a billion cars and trucks, so it's of absolutely no use!
HERE'S a little info that might help you locate the date codes, but remember, 9 could be 1969, 1979 or 1989. There's no way to know for sure. I usually tell people to see if they can find a box close to the date they need and be done with it. Also check out the 69 power steering paragraph. It'll show you the differences (I'm assuming you're looking for a 69, right?)
Ed