user3750: Early blocks contain OP_PUSHBYTES sequences in the coinbase input ScriptSig:

Block 1: OP_PUSHBYTES_4 ffff001d OP_PUSHBYTES_1 04
Block 2: OP_PUSHBYTES_4 ffff001d OP_PUSHBYTES_1 0b
Block 3: OP_PUSHBYTES_4 ffff001d OP_PUSHBYTES_1 0e
Block 4: OP_PUSHBYTES_4 ffff001d OP_PUSHBYTES_1 1a

Do we know why?

user51948: As mentioned here, ffff001d is the "bits" (representing the current difficulty target), and the rest is called the "extra nonce", used to add more possibilities for mining than just the nonce field. This is also discuss here:
ScriptSig (coinbase) structure of the Genesis Block
Later, BIP 34 required that the coinbase include the block height, but this doesn't happen until after block 227,835.

3750: <p>Early blocks contain OP_PUSHBYTES sequences in the coinbase input ScriptSig:</p>
<ul>
<li>Block 1: <code>OP_PUSHBYTES_4 ffff001d OP_PUSHBYTES_1 04</code></li>
<li>Block 2: <code>OP_PUSHBYTES_4 ffff001d OP_PUSHBYTES_1 0b</code></li>
<li>Block 3: <code>OP_PUSHBYTES_4 ffff001d OP_PUSHBYTES_1 0e</code></li>
<li>Block 4: <code>OP_PUSHBYTES_4 ffff001d OP_PUSHBYTES_1 1a</code></li>
</ul>
<p>Do we know why?</p>
 51948: <p>As mentioned <a href="http://en.bitcoin.it/wiki/Transaction#Generation" rel="nofollow noreferrer">here</a>, <code>ffff001d</code> is the &quot;bits&quot; (representing the current difficulty target), and the rest is called the &quot;extra nonce&quot;, used to add more possibilities for mining than just the nonce field. This is also discuss here:
<a href="https://bitcoin.stackexchange.com/q/13122/51948">ScriptSig (coinbase) structure of the Genesis Block</a></p>
<p>Later, BIP 34 required that the coinbase include the block height, but this doesn't happen until after block 227,835.</p>
