Although odd they didn't go up to 'ZZZ' or AVLH ((26⊥⎕A∘⍳)⍣¯1⊢2*15) if you're already using 14 bits in a, I assume, 16-bit machine allowing signed ints

I do like thought that CRXP ≡ (26⊥⎕A∘⍳)⍣¯1⊢2*16

@Silas 26*3 would just be AAA-ZZZ, so 27*3 would be a conservative up rounded estimation