I always thought that both of the ones on the 175 HS and the 200 SS are supposed to be oval to help the rubber carb covers and clutch cable clear the header pipes, the low pipe bikes had round tubes. However, looking at the parts catalog there is a Part Number break at S/N 16K and no P/N difference between the DT/RS or HS/SS as there is for some other specific parts. I take that to mean there was a change to the oval tubes at 16K, maybe for the reasons above, but they didn't want to keep two different part numbers in the system, so all bikes got the oval tubes from then on.
