diff --git a/packages/protocol/contracts/L1/TaikoData.sol b/packages/protocol/contracts/L1/TaikoData.sol
index 72ee38d6e69..6de7242dd12 100644
--- a/packages/protocol/contracts/L1/TaikoData.sol
+++ b/packages/protocol/contracts/L1/TaikoData.sol
@@ -127,7 +127,7 @@ library TaikoData {
         uint96 livenessBond;
         uint64 blockId; // slot 3
         uint64 proposedAt; // timestamp
-        uint64 __reserved1;
+        uint64 proposedIn; // L1 block number, required/used by node/client.
         uint32 nextTransitionId;
         uint32 verifiedTransitionId;
     }
diff --git a/packages/protocol/contracts/L1/libs/LibProposing.sol b/packages/protocol/contracts/L1/libs/LibProposing.sol
index 1b96a0a80ff..7d6d09a44b9 100644
--- a/packages/protocol/contracts/L1/libs/LibProposing.sol
+++ b/packages/protocol/contracts/L1/libs/LibProposing.sol
@@ -160,7 +160,7 @@ library LibProposing {
             livenessBond: _config.livenessBond,
             blockId: b.numBlocks,
             proposedAt: meta_.timestamp,
-            __reserved1: 0,
+            proposedIn: uint64(block.number),
             // For a new block, the next transition ID is always 1, not 0.
             nextTransitionId: 1,
             // For unverified block, its verifiedTransitionId is always 0.