Skeet's pretty much right on. The Speed was released as a lower priced model to reach the mid-range market base, and it introduced WDP's new LP engine. It featured Sensi, an LED board, and IR3 body. The A4 came out a little after that (WDP has this retarded thing where they introduce their lower model first, then their flagship marker after that), used a similar engine, but with a smaller 'noid, a new LCD board, still featured Sensi and a smaller, lighter body. Then, when WDP realized that Sensi was never going to live down the bad rap from COPS, they introduced the Fly, which features break beam eyes to appeal to all the people who refused to buy an angel because they "heard" Sensi/COPS was no good. By the time that was just starting to get real popular, they introduced the Speed 05, which will serve the same purpose as the original Speed, it's their lower priced, stripped down model, and their forthcoming flagship marker will be the G7, to be released later this year.