How do I know which converter box to buy?

Question

We have two 13'' TV's that we need converter boxes for. We purchased one Magnavox TB110MW9 today to see if it works for us, but don't know if it's the right one. Need help before we open it. And, what is the difference with this one and the TB100MW9 ?Thanks for any help. We are in our mid-70's and need all the help possible.