I just got off the phone with a retailer and then with Shure Applications Engineering and neither could tell me what the difference is between the FP1 wireless bodypack transmitter and the SLX1 bodypack transmitter. The reason I'm curious is because the FP1, normally paired with the FP5 bodypack receiver, can also be used with the SLX4 hard-wired receiver, and it costs about $35 less than the SLX1 bodypack receiver. If the specs are the same, why not just use the FP1 with the SLX4 to save money if form factor is not a consideration?