Ok, the SERIAL number was stamped by the inspector, then, who was an Army officer. This Army inspector, if he actually did the stamping, didn't just stamp arbitrary numbers but would stamp the serial production as it was produced in a serial and orderly manner. That's how MGs are tracked and inventoried as with all firearms manufacture.
The Army personnel at Colt were observers, inspectors and participants at times but did not manage the production.
Put it this way, regardless of what they are called, "army control numbers", or whatever, the numbers on the top of the front endcap have absolutely every conventional characteristic of a "serial" number, act as "serial numbers" so it's really a semantic difference, in my opinion. The other stamped numbers have every conventional characteristic of mating or assembly numbers and clearly function in that way so, the duck analogy seems to apply. If it looks like a duck, quacks like a duck, it's a duck.
During those years when Colt produced the Vickers guns, the firearms industry was hidebound by many rigorous manufacturing conventions and the markings on the 1915s follow those conventions, in my opinion, and the logic behind those conventions has not changed.
As a collector, manufacturer and reactivator of MGs for almost 50 years, I have seen, handled and worked on hundreds of different MGs made from 1885 up to current types and some conventions have hardly changed. Markings protocols are one of those conventions that have endured to this day.
I have no argument with Segal, know him and have read his monologues over the years, have gratefully learned from them at times but they are not without factual errors in places, in my experience, and my views will be different due to different perspectives and experiences than his. That's certainly the way life is.
If the inside plate numbers correspond to the other numbers, factory originality is confirmed, which, I believe, is what you want to have confirmed. The registration is legitimate so no worries there.
There is no available listing of the serial numbers of 1915s or other Vickers that I have encountered. Dolf Goldsmith would, without doubt, have included such information in his book on the Vickers if it was available. Maybe it is in his new Vickers book which I have yet to obtain.
When Colt went under for financial reasons many years ago, most, if mot all, of the business and other paper work was thrown out. Some was retrieved by employees, but the records that you want to see apparently no longer exist. We'd all like to see such history for Colt MGs but it, apparently, wasn't worth saving. I have a number of friends who spent years working for Colt and have lamented the loss of that history from just the plain stupidity of those involved. Maybe it was saved but remains undiscovered.
If the anomalies are too overwhelming for what you feel is the value of the gun, just don't buy. Wait for another example that satisfies your needs and perceptions as much as is possible. I've explained what I believe are the facts based on many, many years of experience and research and if my views are not acceptable, ignore them. Simple enough. FWIW