I have a Rider 550 and would like to purchase a second mount for a second motorcycle. I've seen some mention here that the 400 series mounts "are the same" as the 550 mount. Yet the 400 mounts seem to have four contacts, while the 550 mount has two. I cannot find an additional 550 mount listed on the TomTom website for purchase, and the 400 series mount does NOT list the 550 as a compatible model. Can someone confirm that either they are truly compatible (like you've done this and know that it actually is compatible), or not? If not, are 550 mounts available?