Funny Tom, today was the first day that I had a moment to play with toys. When I say toys, I mean my Vetter goodies. So today was Cycle Sound and Vetter Sound research day. I came to the exact same conclusion with the radio units. Each one fits totally different. I mean they all kinda fit, however to perfection is only within one fairing. I have a WJ3, WJ4, WJSS, and about 30 radio units. I played with them all! Wish I had a WJ5 (soon!). Inside of each unit, if it is of its original nature, you will see a little sticker that has the month and the year. I found that each sticker year range fits the fairing year range perfectly. With the WJ3 and lower the 3" speaker version fits perfect I found. More research coming soon!