Thread sizes below 1/4 confusing

Yeah Qld, Australia. We are all metric here but of course you can buy imperial sizes but anything even slightly offbeat and your out of luck. I called about 10 fastner places the the other day and some had never heard of the free spinning washer nut and one guy even said they dont exist. I said they do I have one in my hand. I was at a fastner place yesterday to buy some more 10 - 24 threaded rod and out he comes with it. On the tag it says 3/16 and then says same as 10 -24. Yeah my whole point bloody confusing.