Below is a link to a good description of the difference found by Googling "Cambridge 301".

Bottom line is that a 301 doesn't have an internal GPS while the 302 does. The idea, it appears, is that if the user had an old Cambridge GPS (20, 25, etc) they could upgrade by adding the less expensive 301 and have a complete system (in essence becoming a 302). A standalone 301 would seem to just be a rather smart audio vario.

http://www.easternsailplane.com/east...idge/300Series