Newbie here. I just recently bought a BS 90 Mountain Oil Injected S/N 17B004439 with upswept left side exhaust. According to the parts catalogue, these bikes could have had either left or right side upswept exhaust. What determined which side the exhaust was on? Also, can someone tell me what year model my bike is? My first motorcycle was a BS 50 and that was about 50 years ago. I've owned many different Bridgestones since then. Thanks, Jim P.