Wunwing, Yes it was ABX (Airborne express) with their DC9's. I was one of their certifying engineers in Vancouver Canada. Their containers were the size of door L1 opening and they had a 4" thick solid piece of teflon on the base. You just pushed them around like blocks of ice. Simple trick, well done to the guy who thought of that. At the time in those days, they were buying up used 767's at a great rate, for every 767 they brought and fitted a cargo door to, they had to decommission one DC9 or DC8.
But then DHL pulled out of the deal and I'm not sure what happened to ABX.