The first casper PoC was in 2015, and PoC2 was in early 2016.[1] However, there was a pivot from the early "consensus-by-bet" to the current version, which is based on "prepare and commit" messages. I think the current one is PoC4. [2,3]

1. https://blog.ethereum.org/2016/03/05/serenity-poc2/ 2. https://github.com/ethereum/casper 3. https://github.com/ethereum/pyethereum/pull/791