Weapons on the right side is indeed the commonly accepted reason for the origin of left hand traffic flow, i.e. passing weapon side to weapon side, but I would argue that there are quite a bit more than a “few” modern left-hand traffic countries (see map in previous post), including significantly Japan, many parts of Africa, India, Australia and New Zealand, and Japan was never a British colony. Some continental European countries were also left hand drive originally but changed to right hand drive over time for compatibility with their neighbouring countries. I blame Napoleon.