Not DOT engineers, of course, but ICC. An awful lot of cylinders were making one-way trips then, so it made a lot of sense. After the war, the DOT analyzed results, and found there had been no reliability or safety problems at the higher pressure, so they decided to continue the "+".
There's an interesting postscript to this story that sort of explains why we are still stuck with the nutty and much misunderstood (though the gas industry seems to have no problem with it, the confusion is mostly in the dive world) "+" all these years later. The DOT has on several occasions made attempts to eliminate it, but never quite finished. Some years ago the plan was to, best I can recall, eliminate the plus and make the service pressure of the tanks that now qualify for the plus (3A and 3AA mostly, as far as we are concerned) 2/3s of the test pressure. That is to say, a 2400 psi LP tank which now has a test pressure of 3/5ths of its service pressure, or 4000 psi , and a "+" fill pressure of 2640 would have had a new service pressure of 2667 psi). Tanks would be stamped with both fill and test pressures, as Canadian and Euro tanks mostly are now.
After they'd been working on this for a few years someone pointed out that if they were going to come up with a new specification they really ought to go metric. So they switched to working on a metric version. After they'd been working on the metric version for a few more years someone noticed that the UN had a committee trying to come up with an international "model law" specification, and it occurred to the DOT that maybe it would make more sense to join in with that, rather than come up with another spec that would be good only in the US. Hence the new UN tanks. However, the gas industry turned out to be perfectly happy with things as they were, "+" and all, and not terribly excited about the new UN tanks (which are "high performance" designs, like current SP/exemption tanks), so the DOT allowed them to go on using the old DOT specs . So we still have the plus. I asked the DOT a few years ago if they were every likely to go back and do something about the plus, and was told it was very unlikely as it was pretty much a dead issue.
Historically, the + goes back to WW2 when tanks were in short supply and DOT engineers took another look at their requirements and determined that it would be safe to over fill the normal tank by 10%. Thus immediately increasing the gas storage capacity of the country by 10% without making any new tanks. If anyone has a better/more detailed grasp of historical events concerning the Plus I would like to know more.
