diff --git a/emulators/altirra.yml b/emulators/altirra.yml
index d7e395fa..d8219425 100644
--- a/emulators/altirra.yml
+++ b/emulators/altirra.yml
@@ -378,31 +378,34 @@ files:
     description: "Happy 810 pre-v7 firmware, 4K original (serial $4490)"
     system: atari-400-800
     required: false
-    size: 4096
-    crc32: "942ec3d5"
-    validation: [size, crc32]
-    source_ref: "src/Altirra/source/uifirmware.cpp:528"
+    size: [4096, 8192]
+    validation: [size]
+    on_mismatch: warn
+    source_ref: "src/Altirra/source/uifirmware.cpp:528, src/Altirra/source/firmwaredetect.cpp:186-231"
 
   - name: happy-810-prev7-4k-nonoriginal.rom
     description: "Happy 810 pre-v7 firmware, 4K non-original (serial $1704)"
     system: atari-400-800
     required: false
-    size: 4096
-    crc32: "143b8538"
-    validation: [size, crc32]
-    source_ref: "src/Altirra/source/uifirmware.cpp:528"
+    size: [4096, 8192]
+    validation: [size]
+    on_mismatch: warn
+    source_ref: "src/Altirra/source/uifirmware.cpp:528, src/Altirra/source/firmwaredetect.cpp:186-231"
 
   - name: happy-810-prev7-3k.rom
     description: "Happy 810 pre-v7 firmware, functional 3K inside a 6K image (serial $4B9B)"
     note: >
       The image is 6144 bytes; Altirra identifies it by a checksum taken over
-      the functional 3K, which its firmware list reports as the size.
+      the functional 3K, which its firmware list reports as the size. The
+      checksum skips the hidden 1K of a 4K image and the 2-byte serial, so any
+      serial is recognised; a file it does not recognise can still be given
+      the Happy 810 type by hand.
     system: atari-400-800
     required: false
-    size: 6144
-    crc32: "f81b69cb"
-    validation: [size, crc32]
-    source_ref: "src/Altirra/source/uifirmware.cpp:528"
+    size: [3072, 6144]
+    validation: [size]
+    on_mismatch: warn
+    source_ref: "src/Altirra/source/uifirmware.cpp:528, src/Altirra/source/firmwaredetect.cpp:186-231"
 
   - name: happy-810-v7-6k-modified.rom
     description: "Happy 810 firmware (v7, 6K functional ROM, modified)"
diff --git a/tests/test_profile_checks.py b/tests/test_profile_checks.py
index 8fbca2b0..309bec63 100644
--- a/tests/test_profile_checks.py
+++ b/tests/test_profile_checks.py
@@ -315,5 +315,19 @@ class AppleIIgsRomSizes(_Probe):
                 self.assertFalse(self.accepts(emulator, "ROM", b"\0" * 65536))
 
 
+class AltirraHappy810(_Probe):
+    """ATFirmwareDetection recognises the pre-v7 Happy 810 ROM by its size
+    (3K, 4K or a doubled overdump) and a CRC32 that skips the hidden 1K and
+    the serial; the whole-file CRC32 the profile checked is never computed,
+    so a dump with another serial was refused (firmwaredetect.cpp:186-231 in
+    the 4.40 source)."""
+
+    def test_any_serial_passes_and_size_only_warns(self):
+        self.assertTrue(self.accepts("altirra", "happy-810-prev7-4k.rom", b"\5" * 4096, strict=True))
+        self.assertTrue(self.accepts("altirra", "happy-810-prev7-3k.rom", b"\5" * 3072, strict=True))
+        self.assertFalse(self.accepts("altirra", "happy-810-prev7-4k.rom", b"\5" * 5000, strict=True))
+        self.assertTrue(self.accepts("altirra", "happy-810-prev7-4k.rom", b"\5" * 5000))
+
+
 if __name__ == "__main__":
     unittest.main()