diff --git a/docs/RUGNUX_INTEGRATION.md b/docs/RUGNUX_INTEGRATION.md
index 0bcf2b6c6..5ecc41909 100644
--- a/docs/RUGNUX_INTEGRATION.md
+++ b/docs/RUGNUX_INTEGRATION.md
@@ -58,9 +58,14 @@ H H H    J        Q    K       M    K       M  F    Q    G       L    G       L
 ```
 
 `F` is the French–Wilson amplitude. The header carries the determined space group, the refined cell
-and the wavelength, on a dataset of its own behind the reserved `HKL_base` — so `mtzinfo` and
-`mtzdmp` report the wavelength the data were collected at, and a program that takes *f′*/*f″* from
-the file is given the right edge. The Bijvoet columns are present on any rotation merge, with or
+and the wavelength, on a dataset of its own behind the reserved `HKL_base`, which is where the MTZ
+format puts them. Older rugnux wrote the data on dataset 0, the id reserved for `HKL_base`, and
+CCP4's `mtzinfo` then reported its 1.54187 Å (Cu Kα) default instead of the real wavelength — every
+other reader tried, `mtzdmp`, `truncate`, `ctruncate`, gemmi, iotbx and `phenix.xtriage`, recovered
+the true value from those files as well, so the effect was confined to that one report. Note that
+`<prefix>_unmerged.mtz` still reads 1.54187 under `mtzinfo` and is not wrong: its columns sit on
+`HKL_base` deliberately, as POINTLESS expects, and the wavelength AIMLESS and POINTLESS read is the
+per-batch one, which is correct. The Bijvoet columns are present on any rotation merge, with or
 without `-A`; a stills merge has no Bijvoet split and the file then stops after `F SIGF FreeR_flag`.
 
 There is deliberately **no `DANO`/`SIGDANO`** pair, the anomalous difference columns a CCP4 merged