I recently came into possession of one of the early dive texts by René Bussoz 6th edition - February 1955.
On page 6, I find (emphasis added):
"Constant reserve supply of air (Pre-set)
Maximum breathing safety for the underwater swimmer is ensured, by an air reserve. This supply is brought into use when required by a manually operated lever. Experience has taught that the most reliable signal for warning as to when to use the reserve supply of air, should not depend on devices operated by sound, which are not safe and may rust, nor on so-called leakproof pressure gauges which can give false readings after shock, and whose glass may be broken. Furthermore, there is always the possibility of rupture and leaks in the connections and flexible hose required for connecting such devices to the high pressure air supply. The method used in the "Aqua-Lung" is simpler and more positive. A special arrangement within the cylinder valve causes a restriction to the user's breathing when his regular air supply is nearing exhaustion. This immediately warns him that the reserve air suppy is required, and also, that he must ascend at once. (See page 24. description of the positive reserve.)"
On page 6, I find (emphasis added):
"Constant reserve supply of air (Pre-set)
Maximum breathing safety for the underwater swimmer is ensured, by an air reserve. This supply is brought into use when required by a manually operated lever. Experience has taught that the most reliable signal for warning as to when to use the reserve supply of air, should not depend on devices operated by sound, which are not safe and may rust, nor on so-called leakproof pressure gauges which can give false readings after shock, and whose glass may be broken. Furthermore, there is always the possibility of rupture and leaks in the connections and flexible hose required for connecting such devices to the high pressure air supply. The method used in the "Aqua-Lung" is simpler and more positive. A special arrangement within the cylinder valve causes a restriction to the user's breathing when his regular air supply is nearing exhaustion. This immediately warns him that the reserve air suppy is required, and also, that he must ascend at once. (See page 24. description of the positive reserve.)"
