So i was at Burnsco the other day on the prowl for a new FF. not after a top end unit as im a yak fisho but like the garmin. so i was chatting to the guy about the differences about a couple of units that had different names but appeared to be the same unit. for example the garmin 551c and the garmin 501c. as far as i can see they are the same unit and the guy couldnt answer as to what the difference was. also the echomap 55dv and the 557xs.