That's interesting. After I saw your post I went back and checked and you're right. The first year Heavy-Duti had a separate frame number, but after that they used the same frame as the 3 speed Typhoon (#1920), even though the single and two speed Typhoons were using a different frame (#1919-C). I'm guessing the numbers denoted the different rear fender brackets. And the 1964 American and American Heavy Duty used the same frame (#1919-C), so evidently only the first year of the Heavy-Duti had a special frame. Thanks for pointing that out!
Now I wonder exactly what the differences in the '65 frames were. I'm also wondering if the Typhoon used the #1919-C frame after 1970, when the three speed option was dropped, but the '70's Findley/Waterford catalogs don't show specific part numbers. If they were, then Schwinn was building two different middleweight frames in the '70's, even though neither was three speed. Strange.