diff --git a/domain/sap10_calculator/tables/table_12a.py b/domain/sap10_calculator/tables/table_12a.py
index ab5394f1..b41819b0 100644
--- a/domain/sap10_calculator/tables/table_12a.py
+++ b/domain/sap10_calculator/tables/table_12a.py
@@ -342,11 +342,26 @@ def rdsap_tariff_for_cert(
     # evidence the dwelling is on an off-peak tariff — these charge
     # overnight at the low rate and cannot run economically on a single
     # rate, so the tariff is implied. Direct-acting electric / room heaters
-    # / heat pumps (Rule 3) are NOT off-peak evidence (they run on demand
-    # and exist on single-rate meters too), so they keep STANDARD here
-    # rather than being mis-billed 100% at the off-peak low rate. A
-    # non-electric main also keeps STANDARD (no Rule 4 default — Unknown
-    # must not force off-peak on a gas dwelling).
+    # / heat pumps (Rule 3) keep STANDARD here. A non-electric main also
+    # keeps STANDARD (no Rule 4 default — Unknown must not force off-peak
+    # on a gas dwelling).
+    #
+    # VERIFIED NON-FIX (2026-06-23, "simulated case 48" Elmhurst worksheet):
+    # Elmhurst DOES resolve an Unknown meter + room-heater main (SAP 691) to
+    # 10-Hour Off Peak (its Rule 3) — a hand-built case-48 Summary (main 691,
+    # Unknown meter, 903 dual electric immersion) scores Elmhurst SAP 57; our
+    # STANDARD gives 45, routing Unknown+Rule3 to off-peak gives 55 (7-hour
+    # gives 45 — confirms Elmhurst uses 10-hour). So the "Rule 3 is not off-
+    # peak evidence" intuition is WRONG about Elmhurst's behaviour. BUT
+    # adopting it REGRESSES the lodged-register corpus (72.5%->71.8%, MAE
+    # 0.793->0.827): of 11 Unknown+Rule3 corpus certs only ONE improves
+    # (Apartment 241 -5.38->-1.05); the other 10 overshoot +2.7..+9.1 (Flat 2
+    # +9.11). The register's meter_type=3 certs were lodged with STANDARD-
+    # tariff costing (their low ratings only reconcile at 13.19p flat) — the
+    # gov-API "Unknown" is lossy and does NOT mean off-peak was used. Since
+    # our north star is reproducing the lodged register (not Elmhurst's
+    # deliberate-Unknown worksheet path), KEEP STANDARD. Same "Elmhurst != the
+    # noisy register" family as roof-windows/shutters. Do NOT re-litigate.
     if _meter_is_unknown(meter_type):
         if main_codes & _RULE_1_CPSU_CODES:
             return Tariff.TEN_HOUR