Choice of glyphs

[-JKP-]

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But why construct a glyph set that relies so heavily on numbers and abbreviation symbols?

If it's a cipher, that would not be unusual. Ciphers frequently include number forms. Greek letters are also very popular in medieval and post-medieval ciphers. And, since abbreviations were part of the "alphabet" at the time, a medieval cipher with abbreviation characters would not be out of place either.

I'm not saying it's a cipher, but the choice of shapes would not be out of line for a cipher.