Sub-lexical Regularity in the Voynich Manuscript: BPE Morpheme Signature Discriminates Competing Hypotheses and Reveals a Concentrated Suffix System

The Voynich Manuscript (Beinecke MS 408, c. 1404-1438 CE) resists classification because standard statistical metrics are reproduced by both natural language and human-produced gibberish (Gaskell & Bowern 2022). We introduce Byte-Pair Encoding mean vocabulary morpheme length (BPE VMML) as a writing system classifier and apply it to 21 corpora spanning gibberish, 15 alphabetic languages, two cipher controls, and two logographic references. Three clusters emerge: gibberish (range 2.0-4.1), alphabetic natural and constructed language (4.06-5.25), and high sub-word regularity (5.62-5.92, containing Chinese Pinyin and Voynich). The Voynich VMML of 5.918 (95% CI 5.77-6.05) lies entirely above the alphabetic cluster maximum (4.51), with no overlap. Five mechanistic hypotheses are tested. The central finding is a systematic suffix inventory analysis: 50 distinct 3-character endings account for 80% of the entire Voynich corpus (vs. 172 for Latin, 109 for German). BPE applied to the last four characters of Voynich tokens alone yields VMML=5.868, matching the full-corpus value and exceeding Latin and German full-corpus values. The suffix inventory is stable across scribal sections (Currier A and B share a core of 7 dominant endings). The rank ordering Gibberish < Alphabetic < Voynich >= Pinyin is preserved across all nine BPE parameter configurations tested. All code and data are publicly available. v3.0 revision note: Following a reviewer note (C. Bowern, pers. comm. 2026), EVA digraphs ch and sh are now treated as single glyphs before suffix extraction. The k(80%) for 3-character suffixes is revised from 37 to 50; for 4-character from 110 to 154. Core findings are unchanged. v3.1 (2026-06-13): The alphabetic VMML ceiling reported here (4.51, 21-corpus panel) was subsequently refined to 5.748 using an expanded 71-corpus panel in the companion study (doi:10.5281/zenodo.20633920). Under the expanded panel the Voynich (5.918) remains above the alphabetic ceiling. The central finding — the concentrated suffix system (50 three-character endings covering 80% of the corpus, vs. 172 Latin / 109 German) — is independent of the ceiling estimate and unaffected. A version note to this effect was added at the top of the manuscript.