diff --git a/A00_Common/design/logic/design_calc_registry.ts b/A00_Common/design/logic/design_calc_registry.ts
index f692b269a..958a1ef98 100644
--- a/A00_Common/design/logic/design_calc_registry.ts
+++ b/A00_Common/design/logic/design_calc_registry.ts
@@ -439,8 +439,8 @@ export const CALC_REGISTRY: readonly CalcEntry[] = [
     runner: `py:${B06}B06_Section_Engine_Tilt.py#attach_underdrains`,
     until: "472-4",
     pair: `${B06}B06_Section_Engine_Tilt.py#attach_underdrains`,
-    mirror: [],
-    note: "거울 시험 없음 — 옮기기 전 먼저",
+    mirror: ["test_472_4_underdrain_mirror"],
+    note: "얹기 = GET 상세(파이썬) · 알려진 차 = 옛 underdrain 걷음 TS 만 · 기울임은 cross.design 안",
   },
   {
     id: "cross.fit",
diff --git a/resources/tester/helper_472_4_underdrain_entry.ts b/resources/tester/helper_472_4_underdrain_entry.ts
new file mode 100644
index 000000000..425117255
--- /dev/null
+++ b/resources/tester/helper_472_4_underdrain_entry.ts
@@ -0,0 +1,5 @@
+/* 472-4 C6 거울 헬퍼 — 맹암거 얹기 TS 짝(vite ssr) · test_472_4_underdrain_mirror.py 가 JSON 으로 받음. */
+export {
+  attachUnderdrainSpecs,
+  underdrainSpecsFrom,
+} from "../../common_util/common_util_structure_underdrain";
diff --git a/resources/tester/test_472_4_underdrain_mirror.py b/resources/tester/test_472_4_underdrain_mirror.py
new file mode 100644
index 000000000..947c16d13
--- /dev/null
+++ b/resources/tester/test_472_4_underdrain_mirror.py
@@ -0,0 +1,120 @@
+"""PLAN 472-4 ② C6 (요구 273) — 맹암거 얹기 거울: TS `common_util_structure_underdrain` ↔ 파이썬
+`B06_Section_Engine_Tilt.underdrains_from` · `attach_underdrain_specs`.
+
+못박는 것
+  - 구조물 목록 → 제원(높이 · 경사 · 빈 값 · 0 · 음수 · 글 · 맹암거 아닌 종류) 두 쪽 같음.
+  - 기준 측점(±0.02 m) 얹기 · 얹은 수 같음.
+  - 알려진 차 하나 — 옛 `underdrain` 이 남은 측점: TS 는 걷음 · 파이썬은 둠
+    (서버는 파일에서 새로 읽어 옛 값 없음 · Node 로 옮길 때 한 벌).
+
+    ./venv/Scripts/python.exe -m pytest resources/tester/test_472_4_underdrain_mirror.py -q
+"""
+
+from __future__ import annotations
+
+import copy
+import json
+import shutil
+import subprocess
+from pathlib import Path
+
+import pytest
+
+from B05_Profile.B05_Profile_Structures_Schema import StructureInstance
+from B06_Section.B06_Section_Engine_Tilt import attach_underdrain_specs, underdrains_from
+
+HERE = Path(__file__).resolve().parent
+ROOT = HERE.parents[1]
+
+pytestmark = pytest.mark.skipif(
+    not (ROOT / "config" / "node_modules" / "vite").is_dir(),
+    reason="프론트엔드 의존성 미설치",
+)
+
+
+def _drain(at: float, **options) -> dict:
+    return {
+        "type_id": "cross_underdrain",
+        "placement": "point",
+        "chainage_m": at,
+        "options": options,
+    }
+
+
+STRUCTURES = [
+    _drain(40.0, culvert_height_m=1.2, culvert_slope_pct=3),
+    _drain(80.0, culvert_height_m="", culvert_slope_pct="2.5"),
+    _drain(120.0, culvert_height_m=0, culvert_slope_pct=-1),
+    _drain(160.0),
+    {"type_id": "berm", "placement": "interval", "start_m": 0.0, "end_m": 50.0, "options": {}},
+]
+SECTIONS = [
+    {"chainage_m": 40.01},
+    {"chainage_m": 79.99},
+    {"chainage_m": 120.0},
+    {"chainage_m": 160.03},  # 0.02 밖
+    {"chainage_m": 200.0, "underdrain": {"chainage_m": 200.0}},  # 옛 값 — 알려진 차
+]
+
+
+@pytest.fixture(scope="module")
+def ts(tmp_path_factory) -> dict:
+    work = tmp_path_factory.mktemp("u4724")
+    subprocess.run(  # noqa: S603 — 고정 실행 파일
+        [
+            "node",
+            str(ROOT / "config" / "node_modules" / "vite" / "bin" / "vite.js"),
+            "build",
+            "--configLoader",
+            "runner",
+            "--ssr",
+            "../resources/tester/helper_472_4_underdrain_entry.ts",
+            "--outDir",
+            str(work),
+            "--logLevel",
+            "error",
+        ],
+        cwd=ROOT,
+        check=True,
+        capture_output=True,
+        timeout=300,
+    )
+    bundle = work / "bundle.mjs"
+    shutil.copy(work / "helper_472_4_underdrain_entry.js", bundle)
+    job = json.dumps({"structures": STRUCTURES, "sections": SECTIONS})
+    script = (
+        f"const m = await import({json.dumps(bundle.as_uri())});"
+        f"const job = {job};"
+        "const specs = m.underdrainSpecsFrom(job.structures);"
+        "const count = m.attachUnderdrainSpecs(job.sections, specs);"
+        "console.log(JSON.stringify({specs, count, sections: job.sections}));"
+    )
+    out = subprocess.run(  # noqa: S603
+        ["node", "--input-type=module", "-e", script],
+        cwd=ROOT,
+        check=True,
+        capture_output=True,
+        text=True,
+        encoding="utf-8",
+        timeout=60,
+    )
+    return json.loads(out.stdout)
+
+
+def _python() -> tuple[list[dict], int, list[dict]]:
+    specs = underdrains_from([StructureInstance(**item) for item in STRUCTURES])
+    sections = copy.deepcopy(SECTIONS)
+    return specs, attach_underdrain_specs(sections, specs), sections
+
+
+def test_제원_같음(ts) -> None:
+    specs, _count, _sections = _python()
+    assert ts["specs"] == specs
+
+
+def test_얹기_같음_알려진_차_하나(ts) -> None:
+    _specs, count, sections = _python()
+    assert ts["count"] == count == 3
+    assert ts["sections"][:4] == sections[:4]
+    # 알려진 차 — 옛 underdrain: TS 걷음 · 파이썬 둠
+    assert "underdrain" not in ts["sections"][4] and "underdrain" in sections[4]