Thank you. The scripts sound handy. I’ve downloaded them & I’ll probably use those in the future especially for fonts with pre-existing sets of glyphs but I wanted to go through the process manually for more thorough understanding.
You were correct that the problem was actually in the building of the classes. it was a simple oversight. The “AllLetters” class was missing the forms from the 3 alternates sets: isolated, initial, & final. It included only the default, medial forms so it would fail when the default glyph was substituted by an initial glyph.
With that said I might suggest a slight edit to the tutorial to make it clear that the “AllLetters” (or equivalent) class must contain not only all of the default glyphs but also all of the alternate glyphs. I was a bit thrown off by the line about all of the classes having the exact same number of glyphs in the exact same order. The “AllLetters” class is an exception to that.