alainthailande, as an engineer practicing for even more years than that I have heard of the double zero implementation. Most of the old vacuum tube er empty state electronics computers used it. Sign magnitude is not particularly uncommon until the last 30 years or so. (Gawd - 1980 is THAT long ago! Maybe more like 40 years?)
(Honest, I still have some black hairs..... I'm not all the way white! Suddenly, though, I feel staggeringly old! Um, and when I was a kid the age I am now was considered to be quite old, too.)