Check the top photo on this site, which is the Schwinn- as far as I can tell, the lines of mine are just like that:
http://bicyclesports.us/schwinn_heav..._workhorse.htm
I assume Schwinn changed the lines as they went along. For example, I find some photos of a 1964 Heavy Duty that don't look anything like this.

I find a Craigslist ad that shows: "Vintage Schwinn Heavy-Duti bike (ca 1965)...built as an industrial bike (not the same as the Heavy-Duty)" http://huntsville.craigslist.org/bik/920807539.html

This page shows a recent Schwinn Heavy Duty:
http://www.industrialbikes.com/detail.aspx?ID=3564

Another listing here:
http://www.bikepedia.com/QuickBike/B...Duty&Type=bike

Here's a photo of the "Heavy-Duti" from 1978, different frame. http://oldroads.com/pqdb_img.asp?p=f...ti&mak=Schwinn

I ran across this page showing specifications from 1998. The fork type spelled out matches mine, I think. The style of wheels and rear sprocket do not- which I think is an indication that the wheels are not original.