Two different Schlage Trane thermostats?

The Schlage Trane thermostat lists two different models:

http://link.schlage.com/Products/Pages/Thermostat.aspx

There’s the TZEMT400A and the TZEMT400B. Radio Shack had the former on clearance. Some online sites don’t specify the model number on their web pages. What is the difference between the two?

Thanks.