Thanks for the input! I looked at my Hartland and the yellowing is consistent throughout (except the sanitary hose which seem to still be fairly white). So the Dairy Queen/Tastee Freeze yellowing could be from environmental/production factors. It would be interesting to get a discolored DQ/TF and cut it in half to see the insides. In regards to the Hartland being hollow inside, you mean like this?
http://www.ebay.com/itm/MICKEY-MANTL...cAAOSwX~dWsQXq
I'd never seen one without the hat before, very interesting.