I think that is so you could use them on the K left side feed or XL right side feed...perhaps KR/XR as the Carburetor is on opposite sides for them. Erik K
KR's and maybe some XLR's had them on the left because of leaning left on dirttrack. Not sure if they had a right petcock too. Some guys running flattrack with a stock XLCH tank would put another petcock boss on the left side. I didn't one or two of these for guys back years ago. Ron