OK - I have found the problem.
Somehow there were a bunch of old 3.7 views still on my server. They had the same name but were in different locations.
For example 3.7 has ./Views/ShoppingCart/_DiscountBox.cshtml
Whereas 4.2 has ./Views/Shared/_DiscountBox.cshtml
Because the locations changed, these old files were not overwritten with their newer versions.
Evidently, nopCommerce was picking up the old versions still existing in their old locations before loading the newer 4.2 version locations.
Damn...that was a bugger to track down. I could have sworn I deleted the entire tree structure at the start of all this but...obviously I missed something.
Life is good once again (well...maybe not "good"....but "getting better")