Good guess! But no. Just another ID-10-T. I was using 'file:\\\' instead of 'file:///'. FF made the switch on the sly. Actually, swapped all '\' with '/' which is what threw me. I saw that the 'file:///' matched the 'C:/' in FF, and made the 'file:\\\' match the 'C:\' in the HTML without thinking.
I caught it after comparing the paths between the HTML and the browser after first changing the spaces, and then the spaces and periods to underscores without affecting Calibre. FWIW, I restored the spaces and periods in the path after fixing the 'file:///', and Calibre handled it like a champ!
I appreciate your help, so in closing, let me simply say
