I just purchased this fish finder Model 409190-1KVD. I just noticed that there are two models listed on the rebate form on the website, Model 409190-1KVD and Model 409520-1KVD. When I go to the Humminbird website and look at the products pages I only see Model 409520-1KVD listed though I can do a search and find the other also. Comparing the two I don't see a difference. Anyone know what the difference is between the two models? Thanks