Which block explorer indexes the keys used in mutisig scripts?
You must not see the address in the blockchain as a receiving address. It is a P2SH tx, which means, that the spending condition is sha256’d. This is the type 3address above. The 1-of-1 multisig condition inside defines, that anyone who has the private key to the “1xW3APN...” address can spend this tx, which happened on 2014-07-28 to the target address “1UxOYSY...” 
I highly recommend reading Andreas’ book mastering bitcoin (online available), which explains in nice detail the reasons and insights of those multisig adddresses. 

Blockexplorer blockchain.info obviously doesn’t show the decoded script, cause it is not an address used to receive funds.