The Series-2 holograms are a different design than the Series-3 holograms. Here are some images for each:
Series-2:
Series-3:
And Series-1, for good measure:
Here's a list of denominations and years that carry Series-2 holograms:
2011 1 BTC Brass (most were Series-1, but there are ~500 Series-2)
2012 1 BTC Brass
2012 5 BTC (also found with Series-1 holograms)
2012 10 BTC (both all-silver, and gold-trim)
2011 25 BTC (also found with Series-1 holograms)
2012 1000 BTC
2013 0.5 BTC Brass
2013 0.5 BTC 1/2oz Silver (most were Series-3, but there are ~45 Series-2)
2013 1 BTC Brass
And Series-3 coins:
2013 0.1 BTC 1/4oz Silver
2013 0.5 BTC 1/2oz Silver
2013 1 BTC 1oz Silver (both all-silver, and gold-trim)
I'm releasing a pretty comprehensive site for these coins very soon (tomorrow or over the weekend), so stay tuned!
Edit: Here's the site:
http://www.spotcoins.com/bitcoin/casascius