Why are the single letter spellings (a, e, i, o, u) on the Spelling-Sound Chart for SIPPS Challenge Level but not on the SIPPS Extension Level wall cards?

The students need to see the single-letter spellings of the vowels in Challenge Level because they are taught to read open syllables (syllables that end with a long vowel sound). Students in Extension Level focus primarily on complex vowels; they will already have mastered the short vowel sounds, and they do not yet need to learn the open-syllable conventions.