diff --git a/rugnux/ModelValidation.cpp b/rugnux/ModelValidation.cpp
index 0471551c1..2f6013c4f 100644
--- a/rugnux/ModelValidation.cpp
+++ b/rugnux/ModelValidation.cpp
@@ -146,7 +146,8 @@ ModelValidationResult ValidateAgainstModel(const std::vector<MergedReflection> &
                                            Logger &logger,
                                            const gemmi::SpaceGroup *data_space_group,
                                            bool probe_indexing_ambiguity,
-                                           size_t nthreads) {
+                                           size_t nthreads,
+                                           double wavelength_A) {
     ModelValidationResult result;
     result.model_path = model_path;
 
@@ -730,9 +731,17 @@ ModelValidationResult ValidateAgainstModel(const std::vector<MergedReflection> &
     // --- MTZ of map coefficients so the maps can be re-opened / rebuilt in Coot etc. ---
     try {
         gemmi::Mtz mtz(true);
-        mtz.spacegroup = sg;
+        // The group the REFLECTIONS end up in, which is the model's only where its hand was adopted -
+        // AdoptModelFrame decides this the same way a few lines below. An enantiomorphic pair indexes
+        // identically, so the coefficients are the same numbers either way and only the label moves;
+        // but the two groups have different screw translations, so a reader that expands symmetry out
+        // of this file works in the wrong one if the label disagrees with the .mtz beside it.
+        mtz.spacegroup = result.adopted_model_enantiomorph
+                             ? sg
+                             : (data_space_group != nullptr ? data_space_group : sg);
         mtz.set_cell_for_all(ucell);
         mtz.add_dataset("model_validation");
+        mtz.datasets.back().wavelength = wavelength_A;
         mtz.add_column("FP", 'F', -1, -1, false);
         mtz.add_column("FC", 'F', -1, -1, false);
         mtz.add_column("PHIC", 'P', -1, -1, false);
diff --git a/rugnux/ModelValidation.h b/rugnux/ModelValidation.h
index 0dcc61e94..079683736 100644
--- a/rugnux/ModelValidation.h
+++ b/rugnux/ModelValidation.h
@@ -146,7 +146,8 @@ ModelValidationResult ValidateAgainstModel(const std::vector<MergedReflection> &
                                            Logger &logger,
                                            const gemmi::SpaceGroup *data_space_group = nullptr,
                                            bool probe_indexing_ambiguity = true,
-                                           size_t nthreads = 1);
+                                           size_t nthreads = 1,
+                                           double wavelength_A = 0.0);
 
 // Reindex `merged` into the frame ValidateAgainstModel reported, so the reflection files that are
 // written describe the same indexing as the R-factors and the maps. Returns the space group they are
diff --git a/rugnux/Rugnux.cpp b/rugnux/Rugnux.cpp
index 875d30809..0a46484c4 100644
--- a/rugnux/Rugnux.cpp
+++ b/rugnux/Rugnux.cpp
@@ -4803,7 +4803,8 @@ ProcessResult Rugnux::RunPipeline(RugnuxObserver *observer, bool write_output, b
                                      config_.output_prefix, logger,
                                      data_sg ? &*data_sg : nullptr,
                                      /*probe_indexing_ambiguity=*/config_.reference_data.empty(),
-                                     static_cast<size_t>(config_.nthreads));
+                                     static_cast<size_t>(config_.nthreads),
+                                     experiment_.GetWavelength_A());
             // A model that was asked for and could not be used has to say so where anyone will see
             // it. Without this the run ends successfully with no R-free, no maps and nothing in the
             // report - indistinguishable from a run that was never given --model at all.
diff --git a/rugnux/rugnux_cli.cpp b/rugnux/rugnux_cli.cpp
index b6a8a1e25..d83d204be 100644
--- a/rugnux/rugnux_cli.cpp
+++ b/rugnux/rugnux_cli.cpp
@@ -1790,7 +1790,8 @@ static int RunRugnux(int argc, char **argv) {
                                      output_prefix, logger,
                                      data_sg ? &*data_sg : nullptr,
                                      /*probe_indexing_ambiguity=*/reference_data.empty(),
-                                     static_cast<size_t>(nthreads));
+                                     static_cast<size_t>(nthreads),
+                                     experiment.GetWavelength_A());
             model_validation = validation;
             if (!validation.failure_reason.empty())
                 model_validation_failure = validation.failure_reason;