measure numbers with letters

[Paul Scott]

Why doesn't the following work like 2.19.64 Learning Manual 1.2.5,
Bar Numbers, Selected Snippets, Alternative bar numbering?

Shouldn't I get bars numbered 1, 2, 2a, 2b, 2c?
I've wanted this feature for some time and didn't realize it
existed.

If I get it to work (with someone's help) can I cause the letters to be
capitals?

\version "2.19.64"

\layout {
  indent = 0
  ragged-right = ##t
}

\relative{
  \set Score.alternativeNumberingStyle = #'numbers
  \set Score.barNumberVisibility = #(every-nth-bar-number-visible 1)
  \override Score.BarNumber #'break-visibility = #end-of-line-invisible
  \set Score.barNumberVisibility = #all-bar-numbers-visible
  c''1 \break c \break
  \set Score.alternativeNumberingStyle = #'numbers-with-letters
  c c c 
}

Thank you for any help,

Paul