Part of the afterglow UVB spectrum of the GRB 191004B covering the 912 Å region (rest-frame). Top: 2D spectrum showing the detection of residual emission below the Lyman limit. Bottom: 1D spectrum (black line) with flux density F_λ in units of 10⁻¹⁷ erg s⁻¹ cm⁻² Å⁻¹. Its associated error spectrum (1σ) is in red. The grey shaded region marks the rest-frame range 816−910 Å where residual flux below 912 Å is detected. The solid blue line represents the corresponding average LyC flux density of f₉₀₀ = (2.6 ± 0.2) × 10⁻¹⁸ erg s⁻¹ cm⁻² Å⁻¹ over this range. The spectrum is rebinned at 3 Å per pixel.