If I recall correctly, there was a change over to the 'new style' tank with 'safety valves' and the 'quick connect' (outer thread) valves. This was back in the early 90s, and I remember at the time, it was mostly just an issue of making sure your tank had the outside threads so it would work with your new grill (There was always a sign at the gas station saying they wouldn't swap your tank if it didn't have a pressure relief valve, if it didn't have top and bottom rings etc). Then in the late 90s, the OPD valves came out, and those changed to a triangle shaped knob for the valve. And for a few years, places were offering exchanges to the new safety tanks, and then after a while they wouldn't let you exchange if you had an 'old style', and then finally you couldn't even get the 'old style' refilled.
Now, I suppose it has been so long since the change that some places don't bother to check anymore, and if they get one with the old valve, they probably just chuck it.