If you are looking for the most recent version of the Slingbox itself, this would actually be the Slingbox M2, which has been out for a little over a month. The Slingbox 500 came out in 2012, with the Slingbox M1 coming out between them in 2014. I have linked the Technical specifications below, so that you can compare them from that standpoint as well.
I hope this helps! Please let us know if you have any further questions.