Thank you for this input, Steve. I knew, when I first posted this, that I was entering a realm of colour science that was well beyond my knowledge. I am working on improving that. In the past, I have been successful at learning various concepts on other topics by reading as much as I can find, wrestling with the available information, testing, experimenting and then jumping into a discussion with others who are in the know for better or worse to test what I have learned. This subject, beginning with my original question, as at the top of this thread, has been a real challenge to explore.
After posting the question here, and reading Pats reply, I asked it again in a forum at Cambridge in Colour.com. A great deal of interesting input developed there on this topic and I eventually realised that I had found an answer to my original question. While that answer still seems appropriate, your post here provides a differing description of the nature of the so called Spectrum Locus or, as some members of the Cambridge forum vehemently preferred, Spectral Locus.
As I mentioned in my original post, I had watched, several times, a video made by Mr. Andrew Rodney called Everything you thought you wanted to know about Colour Gamut and RGB Working Spaces My original understanding of the term Spectrum Locus came from that video in which Mr. Rodney referred several times to the horseshoe shaped plot as the Spectrum Locus and the Spectrum Locus as The gamut of human vision and added that Anything that falls outside this particular horseshoe shape is not visible to us and When we talk about gamut this is the big gamut that everything else is contained within. But, if I understand you correctly, here you are saying that the "Spectrum Locus does not, in fact, contain all the colours or hues in human vision.
Im sure there must be something I am missing or about which I’m confused. So, I would appreciate it if you would help me with this, when you have a moment.
Thanks again for your time.
By the way, Id like to take this opportunity to say that ColorThink is an extraordinary tool. As Pat may recall and be able to tell you, I have raved about it to him several times since we first purchased it a few years back. Its amazing. I am so thankful that you developed it. We love it. Cant imagine being without it.