I've been thinking about the right/left headstock question since reading the post last night. From a modern machinery perspective, given that there are a lot more set-up options than a chuck, I can see that pressure would want to be into the holding device on the spindle and not away from it. But that could be accomplished with either a right or left headstock.
When you add cutting threads to the capabilities, it does make sense that since RH threads would be the most common, you would want the toolbit on the operator side of the machine. Yes, you could turn it upside down and cut on the operator side with a LH headstock and the machine turning CCW. But the cutting edge would be hidden from view. Remember, early machines were not production oriented. A minor contributor to it is that centering a tool on a piece is easier with the tool bit cutting edge facing up.
Now if we go way back, to some of the earliest lathes, which I would have to believe were for turning wood, your wood is driving by a driving center that bites into the wood. Therefore, you would want the force of the cutting tool pushing the workpiece into that driving center, not away from it. Yes, there is a tail stock. but if you push hard towards the tailstock the center will simply sink into the wood and the workpiece will loosen.
So, all that said, you could still acomplish the force going into the driving center on a LH headstock machine. But since the majority of people are right-handed, it would be extremely awkward to have full control of a head held tool on a rest using your left arm to feed the tool towards the headstock. So, I also tend to think it is a combination of the way the machine should be operated and the majority of folks being right-handed.
So, what's next, chicken or egg?
