??W2 Econo valves??

So I read the Dulcich article and it basically said there are TWO DIFFERENT versions of the W2 Econo, a standard length valve version that was the original early casting, and another Long valve version that was cast later on. Now that we've pretty much established that there are two different versions, how do I tell which flavor I have? What are the differences? I think mine were cast in '77 judging by the "7" stamped on the end of each head.